chore: remove unused tsconfig.json

[Tony133]

Proposal:

Removes tsconfig.json (and its entry in .npmignore), which is no longer used by anything in the repo.

Why

• The plugin is plain JavaScript (index.js) with hand-written types in types/index.d.ts: there is no build step, no tsc invocation and no checkJs.
• The only consumer of the TypeScript compiler is tstyche (test:typescript). Since tstyche v4, when a file is not covered by a tsconfig.json it applies its own strict default compiler options (strict, exactOptionalPropertyTypes, noUncheckedIndexedAccess, module/moduleResolution: nodenext), which are at least as strict as the config in this repo.
• Most options in the file are emit-related (outDir, declaration, sourceMap, importHelpers, ...) and have no effect, and include references a non-existent ./index.d.ts.
• The file is already excluded from the published package via .npmignore.

Verification

npm test passes unchanged with the file removed: