Add full concurrent multiple language server support

[jclsn]

From what I understand, it is not possible to use all features of two language servers at the same time. You can only disable certain features of one language server and use another language server for these features.

Would it be possible to enable using two language servers at the same time with all of their features? I would for example be able to use clangd and sonarlint-ls at the same time, because Sonarqube is used at our company to improve code quality. I would not want to choose between them.

CoC can run multiple servers and reports in the diagnostic from which source they are coming. Not sure if this is possible in vim9script.

[briancarbone]

I expect the fact only diagnostics are merged would prevent the tailwindcss language server from working simultaneously with the HTML or TSX language servers. I think completion merging would be sufficient to complement diagnostic merging.