Unicode characters

Currently, error_highlight does not handle Unicode characters well. There are two subissues.

1. Ruby::AST::Node#first_column and #last_column seem to return the column in bytes, but String#match handles the index in characters. We need to convert the column indexes.
2. Some Unicode characters are displayed as two (or more?) columns in a terminal with monospace font.

(1) is relatively simple, but (2) is a bit tough. It requires a table telling how many columns each character has. It is known that Reline has such a table. But because error_highlight is a built-in gem that is loaded at Ruby process invocation, it is not good for error_highlight to depend on Reline (unless we make Reline a special built-in gem). We need to discuss how we make the table available to error_highlight.

まず、Ruby::AST::Node#first_column と #last_column が String#match とどのように使用されているかを調べ、次に Reline の lib/reline/unicode/east_asian_width.rb にある Unicode の幅データと比較します。バイトインデックスの場合と端末幅の場合の両方を再現してください。完了には、文字変換について合意されたアプローチを確立し、不適切な依存関係なしで幅データを利用できるようにすることが必要です。