feat: add support for flat configs

[G-Rath]

Something of a revive of #1245, I was looking into upgrading to @typescript-eslint v6 and realised effectively we should move off using the recommended property in meta.docs which in turn means changing how we build our configs and thus "we might as well try doing flat config first".

I'm opening this as a draft for visibility - there's still some follow up to do including updating the docs, actually removing the dynamicness in favor of a static list, and reviewing the latest migration docs to see if there's anything new we need to account for.

Also in particular I'm not thrilled we're listing specific file types in our recommended config now - I wouldn't be surprised if that's easy to change downstream, but I want to look more into that in particular.

@SimenB do you have any feelings about doing a next prerelease vs just landing this? I'll try to test this out but everything's a bit of a mess right now...

Resolves #1408

[github-actions[bot]]

:tada: This issue has been resolved in version 27.9.0 :tada:

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