Support framework-dependent (flat) AL Language extension layout (#2304)

### ❔What, Why & How

Newer, framework-dependent AL Language extensions (for example
marketplace-packaged builds) ship a **flat** internal layout: `altool`,
`alc`, `aldoc`, and the analyzer DLLs all live directly under
`extension/bin`, with no `win32`/`linux` or `Analyzers` subfolder.
Setting such a VSIX via the `vsixFile` setting caused **workspace
compilation** to fail with `Could not find AL tool in the compiler
folder`, even though the same VSIX worked fine for container and
`useCompilerFolder` builds (BcContainerHelper already handles both
layouts).

This PR makes AL-Go's own path resolution layout-aware, mirroring how
BcContainerHelper resolves these tools. Three spots assumed the nested
layout and now fall back to the flat `bin` folder:

- **`Get-ALTool`** (`CompileFromWorkspace.psm1`): probes
`bin/win32/altool.exe` (or `bin/linux/altool`), then falls back to the
flat `bin/altool(.exe)`.
- **`Get-CustomAnalyzers`** (`CompileFromWorkspace.psm1`): URL-based
`customCodeCops` download to `bin/Analyzers` when present, otherwise to
the flat `bin`.
- **`DownloadAlDoc`** (`BuildReferenceDocumentation`): resolves `aldoc`
in `bin/win32|linux` then the flat `bin`.

**Worth noting for reviewers:**
- The `aldoc` fix is pre-emptive. That path is fed by the VSIX bundled
in the BC artifact (not the user's `vsixFile`), so it is not exercised
by the `vsixFile` scenario today, but it carried the identical latent
assumption and was cheap to fix alongside.
- This does **not** cover built-in analyzers (`CodeCop`/`PTECop`/etc.).
Those are passed to `altool` by shorthand name, and `altool` itself
resolves them to `<altoolDir>/../Analyzers`, which is wrong for the flat
layout. That is an `altool` bug (its shorthand resolver is inconsistent
with how it resolves `${CodeCop}`/custom analyzers) and is being tracked
separately against the AL Language / compiler team. The fixes here are
validated end-to-end up to that point: with them applied, a flat VSIX
now gets past tool discovery and fails only on `altool`'s analyzer path.

### ✅ Checklist

- [x] Add tests (E2E, unit tests)
- [x] Update RELEASENOTES.md
- [ ] Update documentation (e.g. for new settings or scenarios)
- [ ] Add telemetry