Adds the code point widths to measure a string

All 5 scenarios hold

The formatter measures alignment in terminal display columns, not in characters. These rules say how wide a value renders in a monospaced font. The Column width feature and the Cell padding and alignment feature build on them.

A string is as wide as its code points together. The rule above says what each code point contributes. A string of square-bodied glyphs is twice as wide as its character count. A string of both kinds is wider than its character count, and less than twice it.

A null string and an empty string both draw nothing. The formatter measures both as zero, and rejects neither.