Create/Use an alternative library for LSP structs

[radeksimko]

Current Version

0.2.1

Use-cases

LS needs a library to represent LSP requests and responses and reflect the reality where clients and servers move at a different pace and hence may not use the exact same version of LSP which could be just strictly unmarshaled.

Attempted Solutions

Currently the LS uses a fork of sourcegraph/go-lsp. The main reason it uses the fork is because it allows us to tolerate missing fields in client requests - which could be missing because the client uses later version of LSP, or just because go-lsp has not codified that field yet - since all structs are maintained manually and often times fields are missing.

As pointed out by @njuCZ this solution however has a flaw. Because go-lsp uses embedded structs, custom unmarshalers may not work as expected and fields will be ignored due to the fact that only unmarshalers of the embedded structs are called, which are unaware of "parent" struct fields - which are in turn ignored.

Proposal

The go-lsp maintainer agreed that a better and more maintainable solution would be generating structs from the spec in the way gopls does it, instead of trying to keep up with the LSP spec.
sourcegraph/go-lsp#8 (comment)

gopls maintainers are hesitant to externalizing https://github.com/golang/tools/tree/master/internal/lsp/protocol just because they can't guarantee a stable API due to the code being generated from TypeScript "spec". However govim maintains their own copy of internal packages at https://github.com/govim/govim/tree/main/cmd/govim/internal/golang_org_x_tools, including the protocol library. I was told that most bugs related to breaking changes are dealt with during the generation phase and most of the remaining ones can be caught via compilation check https://github.com/govim/govim/blob/1b85ff518040c369babbb5a090eefc3fe37bd2f1/cmd/govim/gopls_server.go#L15

With that in mind I believe we could use the same approach as govim.

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