feat: add declaration source maps (.d.ts.map) for CSS Modules

[privatenumber]

Problem

When generateSourceTypes: true, the plugin generates .d.ts files next to CSS Modules. However, "Go to Definition" in editors lands on the generated .d.ts file rather than the CSS source. There's no way to navigate directly to the class selector definition in the CSS file.

Changes

• Add @jridgewell/sourcemap-codec dependency for VLQ encoding
• Create src/plugin/find-class-positions.ts — parses CSS with PostCSS to locate class selector positions (the . character)
• Modify src/plugin/generate-types.ts — generate .d.ts.map source maps that map declare const declarations and export identifiers back to CSS class positions
• Modify src/plugin/index.ts — wire up position finding and .d.ts.map file writing (with skip-if-unchanged optimization)
• Update PostCSS and LightningCSS test suites with source map assertions and new dedicated .d.ts.map tests