chore: move to Eslint flat config

Make the big move to Eslint flat config format. The outcome of Eslint
still should be the same, but some things has changed:
- `eslint-plugin-github` is dropped, flat configs have been out for a
while and most eslint plugins support it, but for no reason and no
activity in sight this plugin is likely not going to support flat config
for a while and to avoid other plugins not being able to update (as they
are requiring flat configs) drop the github rules.
- Nested configs don't work properly and are unified into the root
eslint config, this unification did cause some conflicts and thats why
the `import-x` is in a seperate 'group' to exclude targeting Vue files.
- The `eslint-plugin-i` is deprecated and `esplint-plugin-import-x` is
its succesor which has better support for flat configs, the same rules
are still applied.

The majority of the flat config was generated by
`@eslint/migrate-config` tool.

tools/generate-images.js

@@ -1,6 +1,6 @@
 #!/usr/bin/env node
-import imageminZopfli from 'imagemin-zopfli'; // eslint-disable-line i/no-unresolved
-import {loadSVGFromString, Canvas, Rect, util} from 'fabric/node'; // eslint-disable-line i/no-unresolved
+import imageminZopfli from 'imagemin-zopfli'; // eslint-disable-line import-x/no-unresolved
+import {loadSVGFromString, Canvas, Rect, util} from 'fabric/node'; // eslint-disable-line import-x/no-unresolved
 import {optimize} from 'svgo';
 import {readFile, writeFile} from 'node:fs/promises';
 import {argv, exit} from 'node:process';