Implement `typeHierarchy/subtypes` requests support

[Morriar]

Description

Follow up to https://github.com/Shopify/ruby-lsp/issues/1046 and https://github.com/Shopify/ruby-lsp/pull/2103.

We currently handle textDocument/prepareTypeHierarchy request and typeHierarchy/supertypes requests but the structure of the index doesn't allow for easy descendants lookup.

A partial solution would be to iterate over the whole linearization hash, though since the linearization is lazy, we will be missing entries. It would be better to think about a new data structure to support this feature.

LSP specification: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#typeHierarchy_subtypes.

[github-actions[bot]]

This issue is being marked as stale because there was no activity in the last 2 months