Add additional eslint config file assocations

[terrymun]

eslint@9 adds support for new flat config files, and these new extensions should be reflected in the file associations for eslint.

The new config files are:

• eslint.config.mjs
• eslint.config.cjs
• eslint.config.ts
• eslint.config.mts
• eslint.config.cts

These file names are gleaned from the eslint v9 documentation itself: https://eslint.org/docs/latest/use/configure/configuration-files#configuration-file

[terrymun]

I've reached out to the owners of eslint and they don't have any objections.

If you have time, could you have a quick look at the PR? Thanks @orta 😃