x/pkgsite: Add opensearch / Tab To Search

[Jille]

Having opensearch set up would be great. It allows users to type pkg.go.dev<tab>package name<enter> and go to their package, rather than first waiting for pkg.go.dev to load and then type into the search bar there.

godoc.org had this functionality. I've ported it to pkg.go.dev, I'll send a CL/PR shortly.

/assign Jille

[Jille]

@SnoozeThis https://go-review.googlesource.com/c/pkgsite/+/463986

[SnoozeThis]

(https://snoozeth.is/IRue6A9hYdw) I will wait until https://go-review.googlesource.com/c/463986 is merged and then add a comment.

[hyangah]

I manually set up the search to tab (for example, chrome://settings/searchEngine) but this may be convenient.

cc @golang/tools-team