eslint: use new flat config format (and update deps)

[cstrahan]

This updates eslint (and other dependencies), and switches to the new eslint.config.js flat config format (see https://eslint.org/docs/latest/use/configure/migration-guide).

To preserve the split between .eslintignore and the config file, I've introduced a eslint.ignores.js file, which will be required and included from the generated config, if present.

[cstrahan]

Note that I've changed index.ts (renamed to index.js) to contain the entire config, rather than preserving previous approach where index.ts simply loads and re-exports the config file in the root of the repo.

The reason for this change: typescript-eslint automatically detects the root directory in getTSConfigRootDirFromStack, looking for a file in the callstack with a name matching /^eslint\.config\.(c|m)?(j|t)s$/; so, if index.js requires or imports the config file, then this will make typescript-eslint think that root directory is always the root directory of gts itself.

This is avoided by flipping it around: having gts's eslint.config.js require('./src/index.js').

By having index.js be a javascript file, and requiring that, we ensure that linting the gts repo will work prior to a build (if it was still index.ts, then we'd have to require ./build/src/index.js, which would only exist after compiling).

[cstrahan]

I realized that, in my hastiness, I forgot to commit a couple files; fixed that in the last push.