docs(declaration-files): document implicit export visibility in .d.ts modules

[milcho0604]

Summary

Adds a section to the declaration-files Deep Dive page documenting a
long-standing but undocumented behavior: in a module .d.ts file, all
top-level declarations (types and values) are importable from outside the
module even without the export keyword, and appending export {} opts back
into normal only-explicit-exports visibility.

This is the behavior discussed in microsoft/TypeScript#32182 (open since 2019,
labeled Docs), where it was noted that the handbook doesn't mention it.

Why

I hit this while adding a type-export test to a library PR: a compile-only
test cannot assert the export keyword on a .d.ts type, because removing
export still type-checks for consumers. The behavior is by design, but
nothing in the handbook says so — the Deep Dive page seemed like the natural
home, since it already covers declaration-file name resolution.

All claims verified against TypeScript 5.9.3:

Case	Result
.ts module, import non-exported type	error TS2459
.d.ts module, import non-exported type	OK (no error)
.d.ts module, import non-exported declare const / declare class	OK (no error)
same .d.ts with export {} added	error TS2459

The runtime caution was also verified on Node.js: importing a binding that the
underlying JS module doesn't export throws at load time under ESM and yields
undefined under CommonJS interop.

Notes

• Pure addition at the end of the page; no existing content changed.
• Wording follows the page's existing style ("Say we have a module file
  foo.d.ts… Then consumed it:").

Refs microsoft/TypeScript#32182

[milcho0604]

@microsoft-github-policy-service agree