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.

Start by examining how Ruby::AST::Node#first_column and #last_column are used with String#match, then compare the Unicode width data in Reline's lib/reline/unicode/east_asian_width.rb. Reproduce both byte-index and terminal-width cases; done requires an agreed approach for character conversion and making width data available without an unsuitable dependency.