(Volar 2.5 / mid-Nov 2024) Absorb TS Plugin acceptance tests back into integration tests

[machty]

Volar doesn't have testing support for testing TS Plugins (they only do for old LS-driven diagnostics, i.e. the prior "takeover" approach).

But Volar 2.5 (expected mid-Nov 2024) is supposed to ship with support for testing TS Plugins directly. When this lands, we can dissolve the slow acceptance test cases we built up for testing TS-Plugin-only functionality (e.g. two-file / loose mode components) into integration tests. Also, generally speaking, we can shift more of our classic LS tests into TS-Plugin tests since TS Plugin is the future.

Investigate: maybe there is a way to split testing matrix between LS mode and TS Plugin mode if it makes sense to do this. But then again if we decide that Glint 2 is going to be TS Plugin right out of the gates, then there's less a need for this.