Port upstream Muya cpp alias idempotence

[Renakoni]

Summary

Ports upstream marktext/marktext PR #4816 into the Android Muya copy.

Upstream source: marktext/marktext#4816

This is a follow-up to upstream #4794, so this Android PR is intentionally stacked on Android PR #54 (sync/upstream-4794-cpp-alias). It keeps the one-upstream-PR-to-one-Android-PR mapping while avoiding a misleading PR against main before the alias registration exists there.

Changes

• Makes c++ / h++ alias registration idempotent by adding each alias only when it is absent.
• Prevents duplicated aliases when the Prism module is evaluated multiple times in tests or HMR.
• Extends the alias regression test to reset/re-import the module and assert each alias appears exactly once.

Verification

• pnpm test -- src/utils/prism/__tests__/languageAlias.spec.ts from third_party/muya passed: 5 tests.
• pnpm exec eslint src/utils/prism/index.ts src/utils/prism/__tests__/languageAlias.spec.ts --no-ignore --max-warnings 0 from third_party/muya passed.
• pnpm typecheck from repo root passed.

Audit Decision

This upstream PR is Android-relevant because it hardens the Muya Prism initialization path used by the Android WebView editor. It depends on the alias registration ported in Android PR #54, so this PR targets that branch rather than main.