chore(build): set an explicit rootDir for declaration output

[mrholek]

Preventive — this repo is not currently broken. It hardens the build against a defect that already shipped in the pro packages.

Background

package.json points types at dist/esm/index.d.ts. Emitting the entry declarations anywhere else silently breaks every TypeScript consumer with TS7016 on every import, while the runtime keeps working — the JavaScript entry is unaffected because rollup applies its own preserveModulesRoot: 'src'.

That is exactly what happened to @coreui/react-pro (from 5.27.0) and @coreui/vue-pro (from 5.20.0, reported as coreui/coreui-pro#704): the declarations moved to dist/esm/src/index.d.ts and dist/cjs/src/index.d.ts.

Cause, and why this repo escaped it

It takes two ingredients:

1. @rollup/plugin-typescript 12.x no longer sets rootDir itself.

2. TypeScript 6.x no longer infers the common source directory:

   TS5011: The common source directory of 'tsconfig.json' is './src'.
   The 'rootDir' setting must be explicitly set to this or another path
   to adjust your output's file layout.
   

The pro repos have both. This repo already resolves TypeScript 6.x (transitive dependencies lifted it to the root, where the hoisted plugin picks it up regardless of the typescript@^5.9.3 pin), but is still on plugin 11.1.6, which sets rootDir on its own. So today's output is correct and the next plugin bump would break it.

Fix

Set rootDir explicitly — exactly what TS5011 asks for. This makes the output layout independent of the plugin and compiler versions.

Verification

Built ESM + CJS before and after: output is byte-for-byte the same layout, with the entry declarations at dist/esm/index.d.ts and dist/cjs/index.d.ts and no src/ segment.