compute: define builtin variables for template

Created by: rvantonder

Stacked on https://github.com/sourcegraph/sourcegraph/pull/27885.

This allows to specify some builtins like $repo in the output template, which are substituted for given some arbitrary data type. It does this by replacing recognized builtins like $repo with a Go template version like {{ .Repo }} that is then substituted based on some structs data.