fix: add Yarn PnP support via yarn exec

[SBoudrias]

Problem

The plugin fails in projects using Yarn PnP mode because it hardcodes paths like node_modules/{pkg}/bin/{pkg}. Under Yarn PnP, there is no node_modules on disk — packages are zip-loaded from .yarn/cache/.

Solution

Detect Yarn PnP by checking for .pnp.cjs or .pnp.js at the worktree root, and switch the launch strategy:

• Before: node {worktree}/node_modules/oxlint/bin/oxlint --lsp
• After (Yarn PnP): yarn exec oxlint --lsp

Changes

• src/lsp.rs: Added is_yarn_pnp() helper and get_default_command_and_args() trait method
• src/oxlint.rs: Uses get_default_command_and_args() instead of manually constructing node <path>
• src/oxfmt.rs: Same

Behaviour

Setup	Result
npm / pnpm	node_modules present → existing path logic (no regression)
Yarn PnP	.pnp.cjs present → yarn exec oxlint --lsp
Yarn nodeLinker: node-modules	No .pnp.cjs → existing path logic
No local install	Falls back to extension-bundled binary
Custom binary override in settings	Still applied after detection

[SBoudrias]

I considered adding a config option to pass in a path, but that was more complicated given the bin path isn't detectable... But I do understand this adds a yarn PnP specific understanding to this project; hopefully it's not something you're fundamentally against. 🫰