[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

fuzz/fuzz_targets/red_knot_check_invalid_syntax.rs

@@ -10,7 +10,7 @@ use libfuzzer_sys::{fuzz_target, Corpus};
 use red_knot_python_semantic::types::check_types;
 use red_knot_python_semantic::{
     default_lint_registry, lint::RuleSelection, Db as SemanticDb, Program, ProgramSettings,
-    PythonVersion, SearchPathSettings,
+    PythonPlatform, PythonVersion, SearchPathSettings,
 };
 use ruff_db::files::{system_path_to_file, File, Files};
 use ruff_db::system::{DbWithTestSystem, System, SystemPathBuf, TestSystem};
@@ -112,6 +112,7 @@ fn setup_db() -> TestDb {
         &db,
         &ProgramSettings {
             python_version: PythonVersion::default(),
+            python_platform: PythonPlatform::default(),
             search_paths: SearchPathSettings::new(src_root),
         },
     )