feat: use `worker.mts` for pure ESM

[JounQin]

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Initial checklist

• [X] I read the support docs
• [X] I read the contributing guide
• [X] I agree to follow the code of conduct
• [X] I searched issues and couldn’t find anything (or linked relevant results below)

Problem

A lot of await import in https://github.com/mdx-js/eslint-mdx/blob/3bdd2eb88244aeda636c11f3ccaff35ae2c05ac2/packages/eslint-mdx/src/worker.ts and the hack loadEsmModule

https://github.com/mdx-js/eslint-mdx/blob/3bdd2eb88244aeda636c11f3ccaff35ae2c05ac2/packages/eslint-mdx/src/helpers.ts#L60-L64

is unnecessary.

Solution

Just change worker.ts to pure ESM worker.mts which is just supported by synckit natively.

Alternatives

N/A