[red-knot] Support custom typeshed Markdown tests (#15683)

## Summary

- Add feature to specify a custom typeshed from within Markdown-based
  tests
- Port "builtins" unit tests from `infer.rs` to Markdown tests, part of
  #13696

## Test Plan

- Tests for the custom typeshed feature
- New Markdown tests for deleted Rust unit tests

crates/red_knot_python_semantic/src/program.rs

@@ -108,7 +108,7 @@ pub struct SearchPathSettings {
     /// Optional path to a "custom typeshed" directory on disk for us to use for standard-library types.
     /// If this is not provided, we will fallback to our vendored typeshed stubs for the stdlib,
     /// bundled as a zip file in the binary
-    pub typeshed: Option<SystemPathBuf>,
+    pub custom_typeshed: Option<SystemPathBuf>,
 
     /// The path to the user's `site-packages` directory, where third-party packages from ``PyPI`` are installed.
     pub site_packages: SitePackages,
@@ -119,7 +119,7 @@ impl SearchPathSettings {
         Self {
             src_roots,
             extra_paths: vec![],
-            typeshed: None,
+            custom_typeshed: None,
             site_packages: SitePackages::Known(vec![]),
         }
     }