[red-knot] Statically known branches (#15019)

## Summary

This changeset adds support for precise type-inference and
boundness-handling of definitions inside control-flow branches with
statically-known conditions, i.e. test-expressions whose truthiness we
can unambiguously infer as *always false* or *always true*.

This branch also includes:
- `sys.platform` support
- statically-known branches handling for Boolean expressions and while
  loops
- new `target-version` requirements in some Markdown tests which were
  now required due to the understanding of `sys.version_info` branches.

closes #12700 
closes #15034 

## Performance

### `tomllib`, -7%, needs to resolve one additional module (sys)

| Command | Mean [ms] | Min [ms] | Max [ms] | Relative |
|:---|---:|---:|---:|---:|
| `./red_knot_main --project /home/shark/tomllib` | 22.2 ± 1.3 | 19.1 |
25.6 | 1.00 |
| `./red_knot_feature --project /home/shark/tomllib` | 23.8 ± 1.6 | 20.8
| 28.6 | 1.07 ± 0.09 |

### `black`, -6%

| Command | Mean [ms] | Min [ms] | Max [ms] | Relative |
|:---|---:|---:|---:|---:|
| `./red_knot_main --project /home/shark/black` | 129.3 ± 5.1 | 119.0 |
137.8 | 1.00 |
| `./red_knot_feature --project /home/shark/black` | 136.5 ± 6.8 | 123.8
| 147.5 | 1.06 ± 0.07 |

## Test Plan

- New Markdown tests for the main feature in
  `statically-known-branches.md`
- New Markdown tests for `sys.platform`
- Adapted tests for `EllipsisType`, `Never`, etc

crates/red_knot_python_semantic/src/program.rs

@@ -1,3 +1,4 @@
+use crate::python_platform::PythonPlatform;
 use crate::python_version::PythonVersion;
 use anyhow::Context;
 use salsa::Durability;
@@ -12,6 +13,8 @@ use crate::Db;
 pub struct Program {
     pub python_version: PythonVersion,
 
+    pub python_platform: PythonPlatform,
+
     #[return_ref]
     pub(crate) search_paths: SearchPaths,
 }
@@ -20,6 +23,7 @@ impl Program {
     pub fn from_settings(db: &dyn Db, settings: &ProgramSettings) -> anyhow::Result<Self> {
         let ProgramSettings {
             python_version,
+            python_platform,
             search_paths,
         } = settings;
 
@@ -28,9 +32,11 @@ impl Program {
         let search_paths = SearchPaths::from_settings(db, search_paths)
             .with_context(|| "Invalid search path settings")?;
 
-        Ok(Program::builder(settings.python_version, search_paths)
-            .durability(Durability::HIGH)
-            .new(db))
+        Ok(
+            Program::builder(*python_version, python_platform.clone(), search_paths)
+                .durability(Durability::HIGH)
+                .new(db),
+        )
     }
 
     pub fn update_search_paths(
@@ -57,6 +63,7 @@ impl Program {
 #[cfg_attr(feature = "serde", derive(serde::Serialize))]
 pub struct ProgramSettings {
     pub python_version: PythonVersion,
+    pub python_platform: PythonPlatform,
     pub search_paths: SearchPathSettings,
 }