add go.work and remove Gopkg.toml as Go roots (#6884)

Gopkg.toml was used by dep, Go's original dependency management tool.
It was an experiment that culminated in official and built-in support
for Go modules in mid 2018, and dep was deprecated and archived
in mid 2020 per https://github.com/golang/go/issues/38158.

Now, in 2023, Gopkg.toml files are incredibly rare in actively developed
Go projects, as people use go.mod with Go modules instead.

While here, also add go.work as a root file, since that is used by
Go module workspaces, added in Go 1.18 in early 2022.
gopls or commands like `go build` work inside either go.work or go.mod.
These two root files are the same ones used by gopls integrations with
other editors like vim or neovim.

languages.toml

@@ -292,7 +292,7 @@ name = "go"
 scope = "source.go"
 injection-regex = "go"
 file-types = ["go"]
-roots = ["Gopkg.toml", "go.mod"]
+roots = ["go.work", "go.mod"]
 auto-format = true
 comment-token = "//"
 language-server = { command = "gopls" }