Gopls: Support for Go *.s assembly files

Gopls has rudimentary support for LSP operations in Go assembly files.

Go assembly files have a .s file name extension. LSP clients need to be configured to recognize .s files as Go assembly files, since this file name extension is also used for assembly files in other languages. A good heuristic is that if a file named *.s belongs to a directory containing at least one *.go file, then the .s file is Go assembly, and its appropriate language server is gopls.

Only Definition (textDocument/definition) requests are currently supported. For example, a Definition request on the sigpanic symbol in this file in GOROOT/src/runtime/asm.s:

    JMP ·sigpanic<ABIInternal>(SB)

returns the location of the function declaration in GOROOT/src/runtime/signal_unix.go:

//go:linkname sigpanic
func sigpanic() {

See also issue https://go.dev/issue/71754, which tracks the development of LSP features in Go assembly files.