chore: upgrade templates to TypeScript 6

[NathanWalker]

Bumps typescript to 6.0.3/~6.0.0 across the root and all 5 per-framework templates (react, typescript, vue, svelte, angular).

Fixes real TS6 compatibility issues surfaced along the way:

• Removed deprecated baseUrl from 5 tsconfigs (TS6 hard-errors on this with TS5101).
• Added skipLibCheck to the react template's tsconfig.
• Fixed two strict-nullability issues (typescript and angular templates) surfaced by TS6's stricter defaults.

Verified typechecking cleanly under TS 6.0.3: typescript, react, vue, svelte templates (per-template npm install + typecheck; the root yarn workspace install itself doesn't work in this sandbox — no working yarn/corepack). Angular template's plain tsc passes after fixes.

Known caveats (not fixed in this PR):

• A real Angular AOT build will still reject TS6 until @angular/compiler-cli ships TS6 peer support (currently >=5.8 <5.9) — this is an upstream Angular constraint, not fixable from this repo.
• vue/svelte templates have a pre-existing typeRoots misconfiguration (TS2688) that reproduces identically on TS 5.4/5.7 — unrelated to this bump, left untouched.

Companion PR in NativeScript/nativescript-app-templates upgrades that repo's templates to TypeScript 6 as well.

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