feat(opencode): add built-in Pkl LSP support

[caniko]

Issue for this PR

Closes #

Type of change

• Bug fix
• New feature
• Documentation

What does this PR do?

Adds Pkl as a built-in LSP. OpenCode now recognizes .pkl files, accepts pkl in the built-in LSP config, and starts pkl-lsp --stdio when the command is available.

The built-in LSP docs and focused tests were updated with the new entry.

How did you verify your code works?

• git diff --check
• bun test --timeout 30000 test/config/lsp.test.ts test/lsp/index.test.ts from packages/opencode

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Hey! Your PR title Add built-in Pkl LSP support doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍