Add support for SnippetTextEdits

[jwortmann]

Untested, but I think something like this could work

CI will obviously fail until the protocol types are updated to the latest spec version.

This would probably make https://github.com/sublimelsp/LSP/commit/29100425f8c2635d7b33524519c4bf2bbd6ee86f obsolete at some point in the future, assuming that rust-analyzer switches from its custom SnippetTextEdit to the one from the LSP 3.18 specs.