Improve Go syntax highlighting

[chbk]

Release Notes:

• Improved Go syntax highlighting.

Zed 0.174.6	With this PR

• package_identifier: namespace, language-agnostic scope for modules, packages, namespaces
• method_elem: function.method
• ; ,. ,, ,:: punctuation.delimiter

package my_package
import (
  pkg "fmt"
)
type A interface {
  method_elem(foo int, bar float64) int
}
func main() {
  identifier := true
  const constant int = 3
  for i := 0; i <= 3; i++ {
    pkg.Println(identifier)
  }
}

[maxdeviant]

Thank you!

I removed the @constant.variable change until I can understand the case for it better.