[core] Use type imports for theme augmentation

[Janpot]

These are importing from a .d.ts file, meaning, here is no code to import from, this can confuse build tools such as in #14234. Best to either use .ts files, or use a type import.

Question: Any reason to not just use .ts files? These manual type declarations, while supported, feel a bit like an anti-pattern to me.

[mui-bot]

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[alexfauquette]

Question: Any reason to not just use .ts files? These manual type declarations, while supported, feel a bit like an anti-pattern to me.

@flaviendelangle will have more info, but I think it's just a copy/past from the @mui/lab files that got copy/pasted to other components (tree view, and charts)

https://github.com/mui/material-ui/blob/master/packages/mui-lab/src/themeAugmentation/components.d.ts

This would explain why the data-grid uses .ts file

[flaviendelangle]

@flaviendelangle will have more info, but I think it's just a copy/past from the @mui/lab files that got copy/pasted to other components (tree view, and charts)

It is 😆
If the grid is using a better format and it works for years, then I see no reason not to migrate to their format 👍

[LukasTy]

I agree with the change. 👍
I fear that renaming the files could be a niche breaking change.
We could align the approach and rename files on v8.

Or we can refactor the PR to rename files and merge it to next branch when we start working on it. 🤔

[Janpot]

I fear that renaming the files could be a niche breaking change.

I kind of expect the type generation to have the exact same output. Will test.

[Janpot]

Looks like the difference in output is:

• this PR:
  output declaration files re-export types with export type, instead of just export. No js modules are generated
• renaming the files:
  output declaration files re-export with just export, just like we currently do. But it also generates empty js modules

I don't see how either of these could lead to a breaking change for the user. The .ts method already works for the data grid anyway.

[Janpot]

It looks like since I made this PR, they have been updated already on master. Closing this for now, X repo satisfies the needs for #14234 now. Up to the team if they want to align further.