[1.x] fix: resolve regression in TS typing errors

[DavideIadeluca]

Resolves regression introduced in #4027

Fixes #0000
If a shims.d.ts declaration file doesn't include any import statement, the TS compiler will treat it as an ambient declaration — a type-only interface with no associated value at runtime — which causes regression in other parts of the codebase (notably extend.ts).

Changes proposed in this pull request:

• Add dummy imports to shims to work around TypeScript's module augmentation mechanism.

Reviewers should focus on:

• For consistency reasons, I always included all module declaration imports, even if only one or no additional were required.

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)