feat: implement new lint prefer-ascii

[magurotuna]

This PR implements a new lint rule prefer-ascii that makes sure that the code is composed of only ASCII characters.

Rendered doc: https://github.com/magurotuna/deno_lint/blob/ace535166b4f42a603f9ff5c540d104b4e932647/docs/rules/prefer_ascii.md

In addition, moves display_diagnostics function into diagnostics.rs and adds test cases. The previous logic of this function didn't take multi-byte characters into account, which has been fixed as well. However, to properly display diagnostic annotations for double-width characters, we still need to wait for my patch to land: rust-lang/annotate-snippets-rs#46
With the current version of annotate-snippets, displayed diagnostics may point to the wrong position if the snippet contains double-width characters, like:

Closes #832

[bartlomieju]

LGTM! Thanks!