[imorland]

Fixes: no issue.

Changes proposed in this pull request:
Flarum 2.0 builds and checks its typings with TypeScript 4.9: core's ^4.5.4 resolves to 4.9.5. The upgrade now pins an extension's typescript to ^4.9.5, like the scaffold, and says why when it comes down from TypeScript 5.

It also handles what TypeScript 4.9 can't read:

• it drops the tsconfig.json compiler options and values 4.9 doesn't have, using a list generated from typescript@4.9.5 itself, and reports what it removed;
• it flags an extends list;
• it puts a @TODO above syntax 4.9 can't compile: const type parameters, using, export type * and import attributes. TypeScript sources are only searched line by line, never parsed or reformatted.

post-build-typings now works with macOS's BSD sed as well as GNU sed, in the upgrade and the scaffold. JSON with a trailing comma before ], which tsconfig.json allows, no longer fails the step.

Reviewers should focus on:

• Every flagged syntax, and every tsconfig rule, was checked against typescript@4.9.5 rather than release notes. That ruled out accessor, satisfies and import … assert, which 4.9 has.
• Checked across 9,313 real TypeScript files (core, bundled and installed third-party extensions): no false positives.
• Checked across 133 real tsconfig.jsons: only fof/links and fof/polls change, losing moduleResolution: "bundler". Six FoF configs with a trailing ,] used to fail the JSON parse; they now load.

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?
• 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. (The built CLI ran step 2 on a 1.x-style fixture extension, and the changes and the commit came out as intended.)
• Frontend changes: tests are green (run yarn test in js/). (yarn test: 244 tests, plus Prettier, ESLint and tsc.)
• Frontend changes: tests have been added, or are not appropriate here. (23 new tests, written first and seen failing. One runs the new post-build-typings for real, so CI checks it with GNU sed.)
• 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.

Required changes:

None.