Split copy-res into two phases during yarn start

[Johennes]

Requires: #26591

copy-res.ts -w currently runs in parallel with the Webpack build which causes the following problems:

1. When yarn starting after yarn clean the language file is usually not generated in time for Webpack to process it which breaks the build
2. Files are being copied even without changes after setting up the watches which triggers another Webpack build

This pull request rearranges copy-res.ts -w so that it only writes files after actual changes being triggered on the watches. This makes it necessary to call the script once without the -w flag before to perform the initial copying which is what's used to perform the initial copying before starting the Webpack build on yarn start. Additionally the initial copy-res is ran in parallel with the module system build and the Jitsi download to gain some more slight performance.

With this change a clean yarn start takes ~21s on my machine. Before, a clean yarn start failed and subsequent yarn start took about the same time.

Unfortunately, even with this change, I'm still often seeing a secondary Webpack build after yarn starting.

2023-11-15 21:54:25.630 [element-js]    2545 modules
2023-11-15 21:54:31.126 [element-js]    2545 modules

There doesn't seem to be any other file generation after the first phase. So I'm unsure what triggers this. It might be similar to what's mentioned in https://github.com/vector-im/element-web/blob/develop/scripts/copy-res.ts#L169 but sleeping doesn't feel like a great solution.

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This change is marked as an internal change (Task), so will not be included in the changelog.