feat: add typescript type declarations for analyzeCommits

[Mearman]

This package doesn't ship any TypeScript types right now — no .d.ts file, no types field in package.json — and there's no @types/semantic-release__commit-analyzer on npm either, so anyone calling analyzeCommits from TypeScript gets an implicit any for the whole module.

This adds an index.d.ts typing analyzeCommits(pluginConfig, context) and its return value. The shapes come straight from the JSDoc already on analyzeCommits in index.js and on the loader functions in lib/, plus the options table in the README (presetConfig in particular isn't in the JSDoc but is documented there). ReleaseType and AnalyzeCommitsContext are imported from semantic-release's own published types rather than redeclared, since anyone using this plugin already has semantic-release installed as a peer dependency.

I also had to change exports from the bare "./index.js" string to an object with a types condition — under moduleResolution: bundler/node16/nodenext, TypeScript only looks for a declaration file through a types condition inside exports once exports exists at all, it doesn't fall back to the top-level types field. I kept the top-level types field too for anyone still on classic resolution, and added index.d.ts to files so it's actually in the published tarball.

I checked this actually resolves correctly (not just "looks right") by packing the module with npm pack, installing the tarball into a scratch project, and compiling a small consumer file against it under bundler, node16, nodenext, and classic node/node10 resolution — all four pick up the types correctly, and @ts-expect-error on a couple of intentionally-wrong calls (missing pluginConfig, a ReleaseRule missing release) confirms it's real type-checking and not just resolving to any. npm test and publint --strict both pass unchanged.