pkg/river: introduce scanner package

This commit introduces the lexer for River (grafana#1839). It is
not wired in anywhere, and is the first of several PRs to slowly bring
over functionality from the prototype.

Note that this commit does not indicate we will ship River, just that we
are continuing the prototyping process in the main branch. pkg/river may
still be removed in the future in favor of an alternative solution.

Author: rfratto