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.

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