[imorland]

Fixes #0000 (follows flarum/framework#5120: Flarum builds with TypeScript 5.9 from 2.1, and flarum-webpack-config 3.1.0 supplies it to extensions)

Changes proposed in this pull request:
fl upgrade 2.0 brought every extension's TypeScript down to ^4.9.5, and fl init scaffolded it, which now puts extensions back into the @types/node parse errors of flarum/framework#5120. Both now use ^5.9.3, with flarum-webpack-config ^3.1.0 and @flarum/jest-config ^2.1.0. A TypeScript 6 or newer still comes down to 5.9, because flarum-tsconfig's moduleResolution: "Node" fails under it, unless the extension's tsconfig already resolves modules another way and sets no baseUrl. tsconfig options are now checked against what TypeScript 5.9 accepts (generated from typescript@5.9.3, minus the options 5.5 removed), and TypeScript 5 syntax is no longer flagged.

Reviewers should focus on:

• Run over the 131 real extension tsconfigs on my machine: TypeScript 5.9 accepts every result. 83 move to ^5.9.3, 45 without TypeScript are left alone, and 3 already working on TypeScript 7 (fof/polls, fof/links) are kept.
• The "types": [] the upgrade adds is unchanged; only its comment, which blamed TypeScript 4.9, is corrected.

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, CLI.)
• 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/). (N/A, CLI.)
• Frontend changes: tests have been added, or are not appropriate here. (N/A, CLI.)
• Backend changes: tests are green (run composer test). (yarn test: 421 tests, Prettier, ESLint and tsc.)
• Backend changes: tests have been added, or are not appropriate here.
• 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.