[imorland]

Fixes: no issue.

Changes proposed in this pull request:
More of what real 1.x extensions' upgrades got wrong, one commit each:

• TypeScript that exports from an ambient declare module block failed to parse, which stopped the formatting, admin and misc steps.
• ext: imports: every reference to an extension's module is rewritten, in export … from, import(), require() and declare module too. The tsconfig keeps the path where the 1.x mapping found the extension's typings.
• A method assigned to a lazy module's prototype became an extend() that never replaced it. It is now an override().
• A new extender went into every array extend.php returns, including a Conditional's. It now goes only into the outermost one.
• Search: a class 1.x registered as both a gambit and a filter is now registered once, and only constrain()'s query parameter becomes an Eloquent builder. A key-value gambit gets its hint translation and a @TODO to write it.
• flarum-webpack-config ^3.0.7 (fix(webpack-config): register a default exported expression as its value framework#5234) for upgraded and new extensions.

Reviewers should focus on:

• src/utils/babel-ts.ts: Recast's parser with Babel's undeclared-export check turned off. Other parse errors still fail the step.

Screenshot
N/A

Necessity

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

Confirmed

• Frontend changes: tested on a local Flarum installation. (N/A, CLI.)
• Frontend changes: tests are green (run yarn test in js/).
• Frontend changes: tests have been added, or are not appropriate here.
• Backend changes: tests are green (run composer test). (N/A)
• Backend changes: tests have been added, or are not appropriate here. (N/A)
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite). (N/A)
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.