Implement isort's default-section setting (#10149)

## Summary

This fixes https://github.com/astral-sh/ruff/issues/7868.

Support isort's `default-section` feature which allows any imports that
match sections that are not in `section-order` to be mapped to a
specifically named section.


https://pycqa.github.io/isort/docs/configuration/options.html#default-section

This has a few implications:

- It is no longer required that all known sections are defined in
`section-order`.
- This is technically a bw-incompat change because currently if folks
define custom groups, and do not define a `section-order`, the code used
to add all known sections to `section-order` while emitting warnings.
**However, when this happened, users would be seeing warnings so I do
not think it should count as a bw-incompat change.**

## Test Plan

- Added a new test.
- Did not break any existing tests.

Finally, I ran the following config against Pyramid's complex codebase
that was previously using isort and this change worked there.

### pyramid's previous isort config


5f7e286b06/pyproject.toml (L22-L37)

```toml
[tool.isort]
profile = "black"
multi_line_output = 3
src_paths = ["src", "tests"]
skip_glob = ["docs/*"]
include_trailing_comma = true
force_grid_wrap = false
combine_as_imports = true
line_length = 79
force_sort_within_sections = true
no_lines_before = "THIRDPARTY"
sections = "FUTURE,THIRDPARTY,FIRSTPARTY,LOCALFOLDER"
default_section = "THIRDPARTY"
known_first_party = "pyramid"
```

### tested with ruff isort config

```toml
[tool.ruff.lint.isort]
case-sensitive = true
combine-as-imports = true
force-sort-within-sections = true
section-order = [
    "future",
    "third-party",
    "first-party",
    "local-folder",
]
default-section = "third-party"
known-first-party = [
    "pyramid",
]
```

crates/ruff_workspace/src/options.rs

@@ -2099,6 +2099,16 @@ pub struct IsortOptions {
     )]
     pub section_order: Option<Vec<ImportSection>>,
 
+    /// Define a default section for any imports that don't fit into the specified `section-order`.
+    #[option(
+        default = r#"third-party"#,
+        value_type = "str",
+        example = r#"
+            default-section = "third-party"
+        "#
+    )]
+    pub default_section: Option<ImportSection>,
+
     /// Put all imports into the same section bucket.
     ///
     /// For example, rather than separating standard library and third-party imports, as in:
@@ -2226,6 +2236,9 @@ impl IsortOptions {
         if no_sections && self.section_order.is_some() {
             warn_user_once!("`section-order` is ignored when `no-sections` is set to `true`");
         }
+        if no_sections && self.default_section.is_some() {
+            warn_user_once!("`default-section` is ignored when `no-sections` is set to `true`");
+        }
         if no_sections && self.sections.is_some() {
             warn_user_once!("`sections` is ignored when `no-sections` is set to `true`");
         }
@@ -2241,6 +2254,10 @@ impl IsortOptions {
         let mut section_order: Vec<_> = self
             .section_order
             .unwrap_or_else(|| ImportType::iter().map(ImportSection::Known).collect());
+        let default_section = self
+            .default_section
+            .unwrap_or(ImportSection::Known(ImportType::ThirdParty));
+
         let known_first_party = self
             .known_first_party
             .map(|names| {
@@ -2344,24 +2361,13 @@ impl IsortOptions {
             }
         }
 
-        // Add all built-in sections to `section_order`, if not already present.
-        for section in ImportType::iter().map(ImportSection::Known) {
-            if !section_order.contains(&section) {
-                warn_user_once!(
-                    "`section-order` is missing built-in section: `{:?}`",
-                    section
-                );
-                section_order.push(section);
-            }
-        }
-
-        // Add all user-defined sections to `section-order`, if not already present.
-        for section_name in sections.keys() {
-            let section = ImportSection::UserDefined(section_name.clone());
-            if !section_order.contains(&section) {
-                warn_user_once!("`section-order` is missing section: `{:?}`", section);
-                section_order.push(section);
-            }
+        // Verify that `default_section` is in `section_order`.
+        if !section_order.contains(&default_section) {
+            warn_user_once!(
+                "`section-order` must contain `default-section`: {:?}",
+                default_section,
+            );
+            section_order.push(default_section.clone());
         }
 
         Ok(isort::settings::Settings {
@@ -2394,6 +2400,7 @@ impl IsortOptions {
             lines_between_types,
             forced_separate: Vec::from_iter(self.forced_separate.unwrap_or_default()),
             section_order,
+            default_section,
             no_sections,
             from_first,
             length_sort: self.length_sort.unwrap_or(false),