Add language server support for C files

[valstu]

We need to build language server which supports C language so that we can show proper errors and warnings before compiling code.

[vbar]

We decided to go with clangd, which is the LLVM C++ and C language server, i.e. using the same compiler backend as the wasm compilation. It's true we don't need the C++ support, but that's disabled automatically for source files with .c extension. Another missing piece is that clangd doesn't have a network interface (apparently most language servers don't), but a WebSocket proxy can be added for that - probably https://github.com/wylieconlon/jsonrpc-ws-proxy .

[vbar]

A WS proxy calling clangd had been integrated into into the compilation service.