Fix global typings for extensions

[davwheat]

Fixes #2963

Changes proposed in this pull request:

• Adds support for our global typings in extensions
• Linked tsconfig PR sets up this configuration
• Drops global app typings (people should import instead)
• Adds common namespace app export
• Adds missing app imports to core
• Converts Mithril imports within core to type imports (causes error if we attempt to use the imported mithril for anything except types as this causes duplicated code)
• Adds typings for global flarum variable

Reviewers should focus on:

To test this locally...

1. Require this branch in an extension's composer.json
2. composer update
3. cd vendor/flarum/core/js
4. npm i
5. npm run build-typings
6. Modify extension's tsconfig.json as per Add configuration for global typings flarum-tsconfig#2
7. Test global typings! (dayjs, mithril)

Screenshot

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

Required changes:

• Related documentation PR: Use new tsconfig.json contents docs#358
• Related core extension PRs: Add configuration for global typings flarum-tsconfig#2

[askvortsov1]

Not tested locally, but generally LGTM. Solid set of changes!

Left a few nits.

[SychO9]

Weird 🤔, compiling with this seems to break extensions that use TS but don't import app.
app points to flarum.core.app.

Uncaught TypeError: Cannot read property 'addTranslations' of undefined
Uncaught TypeError: flarum.core.app.load is not a function

[davwheat]

Hmm... Typing changes couldn't do that as that's handled by the webpack config... Which extension is this?

[SychO9]

never mind, it doesn't have anything to do with extensions, just checking out this branch and building dist files results in the error above. could it be the ../common/app imports ? how does that point to Application ?

[davwheat]

It doesn't. In my testing, returning window.app seemed to work fine. Might have to revisit that in that case.

My thinking was that the app instances are created by the forum/admin exports, and overwriting them with a Common app would be disastrous.