x/tools/gopls: create gopls "experimentalFeatures" setting

[stamblerre]

@findleyr suggested that some users may want to stay on the bleeding edge of our new functionality, but likely find it annoying to switch on the different new flags as we add them. We should create a new "experimentalFeatures" flag that we update as we add new behavior and opts users into all of the experimental features.

[gopherbot]

Change https://golang.org/cl/260002 mentions this issue: internal/lsp: add support for an "enable all experiments" settings