fix: switch to eslint-plugin-import-x and drop fixup wrappers

eslint-plugin-import still lacks native flat config support, forcing
@eslint/compat fixupPluginRules wrappers. Swap to eslint-plugin-import-x
(maintained fork with native flat config) and drop the fixup wrappers
for jsx-a11y (v6.10+) and react (v7.37+) which also ship native flat
config now.

Also scope the Next.js config block to JS/TS files so rules referencing
the import plugin don't try to apply to JSON/YAML files and fail with
"could not find plugin 'import'".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: JoshTheWanderer