Support ESLint 9 flat config

[predragnikolic]

closes #68

[predragnikolic]

There are 2 small things: (non of them I see as blockers)

1. JSON schema for useFlatConfig can be more specific microsoft/vscode-eslint#1885

The default for useFlatConfig is null, but the type in the schema is a boolean.
This point isn't that import, because we do not specify this setting. The server will be able to understand flat configs, as long as someone has ESLint version higher than 8.57.0 (v9.x.x and above). See https://github.com/microsoft/vscode-eslint/pull/1882/files

2. Should plugins still have eslint.experimental.useFlatConfig in their settings? microsoft/vscode-eslint#1886

While working on this PR, I wanted to remove the eslint.experimental.useFlatConfig

LSP-eslint/LSP-eslint.sublime-settings

Line 21 in 1e61688

"experimental.useFlatConfig": false,

but doing that will cause the server to throw errors like Cannot read properties of undefined (reading 'useFlatConfig') so I decided to keep the eslint.experimental.useFlatConfig setting for now.

I've opened an issue(question) just to see what their answer is.