add autofix script to `package.json`

[AprilSylph]

Suggestion from https://github.com/AprilSylph/XKit-Rewritten/pull/1266#issuecomment-1707330847

npm run autoformat should execute eslint src/ --fix.

[marcustyphoon]

"autoformat" as a keyword for eslint seems a little odd in an ecosystem in which prettier and dprint exist, but in the context of this project I think it's fine.