Upgrade to eslint 9

[dhess]

This version switches to a new "flatconfig" config format that is incompatible with the previous config format. It's going to take quite a bit of work to port our eslint config, and at the moment, it appears that several of the plugins we use don't support the new format. There are ways around that, but it's probably not worth the effort — let's wait until the dust settles.

Ref:

• https://eslint.org/docs/latest/use/configure/migration-guide#start-using-flat-config-files
• https://stackoverflow.com/questions/74237042/how-to-correctly-configure-the-parser-plugins-with-eslints-new-flat-config
• https://news.ycombinator.com/item?id=39972086