fix(reactivity): export MarkRaw type

[sxzz]

closes #7048

Reproduction: https://github.com/sxzz/vue-issue-markRaw

src/main.ts:3:14 - error TS4023: Exported variable 'foo' has or is using name 'RawSymbol' from external module "/path/vue-issue-markRaw/node_modules/.pnpm/@[email protected]/node_modules/@vue/reactivity/dist/reactivity" but cannot be named.

3 export const foo = markRaw({ key: 'value' })
               ~~~

[posva]

There seem to be more types that are being reported as missing by api-extractor.

I wonder if some of these should be exported but prefixed with an _ and marked as @internal so we don't expose types that aren't meant to be used in userland like this one.