31.13.0

feat: highlight PSL mixins (#2011)

Adds TextMate syntax highlighting for the mixin syntax supported by
Prisma 8.

## Changes

- Highlight mixin declarations, including declarations inside
namespaces.
- Highlight `+Mixin` and `+namespace.Mixin` inclusions.
- Add regression tests for declaration bodies, existing identifiers,
comments, and signed numbers; document the grammar test command.

## Why

Reuse existing model and enum body rules so mixins retain the same field
and attribute highlighting as ordinary declarations.

**Scope:** TextMate grammar, tests, and testing documentation only; no
language-server changes.

**Verification:** `pnpm --dir packages/vscode test tests/grammar` — all
52 tests pass. Changed code has no LSP diagnostics.