Properly handle oneOfs in schemas

[tamasfe]

Currently all oneOfs are merged for autocompletion, instead only the relevant ones should be shown to the user.

[Bludator]

I think that this not only gives merged autocomplete result but also results in fails in validation:

With this setup

VSCode extension:

Expected the value to be exactly one of "test1" or "test2", but it is valid for "test1" and "test2"

or CLI:

> npx @taplo/cli lint -s test-schema.json test.toml

npx: installed 20 in 5.21s
error: failed schema validation
 --> test.toml:1:1
  | 
1 | testProp = 10
  | ^ expected exactly one schema to match, but 2 schemas matched

failure: processed 1 document with 1 error

[tamasfe]

Yes, schemas probably need an overhaul along with #136 as well.

[Profpatsch]

I would like to use oneOf in our schema as well, but as it stands it looks like the usability of hover-documentation suffers a lot when overlapping oneOf is involved.

[Profpatsch]

For example this trick: https://gist.github.com/Profpatsch/af40fc52f51cc94796e05bf7fed395ed doesn’t lead to helpful error messages when used.