Add custom LSP server

[julfarn]

Editor support for .mlir files is limited because the LSP does not know about our custom dialects.

Building an LSP server that includes new dialects seems to be fairly straightforward: https://mlir.llvm.org/docs/Tools/MLIRLSP/#supporting-custom-dialects

The devcontainer config should be updated to build this tool at appropriate times and point to it for IDEs to use.

[rainij]

If it is really as simple as you say (including maintenance) then I am in favor of adding it. But it really has to be simple as I do not have the feeling that it is as important as for normal code (our test files are usually extremely small).