transformer: add style guide for import order

[Dunqing]

As #6582 does, we can write the import order style guide here. This way can help contributors how write transformer plugins that conform to our style.

oxc/crates/oxc_transformer/README.md

Lines 21 to 22 in 03e033f

	## Style guide for implementing transforms

[overlookmotel]

Personally, I think it'd be ideal if we used this same import order convention in all crates, not just transformer.

So we'd need a universal "style guide". Where would that go? https://oxc.rs/docs/contribute/rules.html?

Or, even better, can we do it automatically with rustfmt/clippy/autofix action?

[Dunqing]

Or, even better, can we do it automatically with rustfmt/clippy/autofix action?

Yes, please! I can't remember this order.

[overlookmotel]

That was a question, not a statement! I don't know if any of these tools support doing that.

[camc314]

one of these ?
https://rust-lang.github.io/rustfmt/?version=v1.6.0&search=#group_imports /
https://rust-lang.github.io/rustfmt/?version=v1.6.0&search=#reorder_imports

[overlookmotel]

Thanks @camc314. group_imports with StdExternalCrate option gives us some of what we want, and would be an improvement. But:

1. It's unstable. Do we run rustfmt in nightly mode?
2. It wouldn't enforce oxc_* crates being grouped together after external crates.