Use flat config for the project's eslint configuration

[BPScott]

Convert the project's eslint config to the flat style.

This shall let us dogfood the recommended eslint config when we add it.

This is a 1:1 migration, with the exception of adding .github/ISSUE_TEMPLATE.md to the ignores. I'm not sure what I've managed to do here, but it seems we weren't linting that before but now we are.

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[JounQin]

@BPScott .md files are linted by ESLint through remark-lint integration. And eslint-plugin-mdx supports flat config at the same time.

See also https://github.com/mdx-js/eslint-mdx/tree/master/packages/eslint-plugin-mdx#flat-config, https://github.com/mdx-js/eslint-mdx/tree/master/packages/eslint-plugin-mdx#mdxremark and https://github.com/prettier/eslint-plugin-prettier/blob/master/.remarkrc

[JounQin]

Please add mdx plugin back.

[BPScott]

Gotcha, I hadn't clocked that eslint-plugin-mdx also lints against the markdowny portions of the markdown file rather than just the codeblocks. I'll add it back.

[BPScott]

There we go, I believe that's the markdown stuff put back properly. That last rebase was tidying up the wording of the global ignores block at the bottom and some variable renaming. @JounQin I've tagged you for one last re-review if you'd be so kind.