Flat config support

[rijenkii]

I see that it has been mentioned a couple of times (#59 (comment), #48 (comment)), it is still not supported.

[rijenkii]

Bump, now that eslint-plugin-vue supports flat configs: https://github.com/vuejs/eslint-plugin-vue/releases/tag/v9.24.0.

[mwkcoding]

This should happen ASAP as the eslintrc config is deprecated.

[bmulholland]

Flat Config solves the ESLint sub-dependency limitation noted in the README, so that section should be updated when the plugin is.