Add support for the capitalize text transform

[mojavelinux]

In addition to the uppercase and lowercase transform for text, add support for the capitalize transform as defined in CSS. This transformation is controlled by the text-transform property in the theme.

According to MDN, the rules for the capitalize transform are as follows:

Convert the first letter of each word to uppercase. Other characters remain unchanged. A letter is defined as a character that is part of Unicode's Letter or Number general categories. Any punctuation marks or symbols at the beginning of a word are ignored.

We can leverage the String#capitalize method in Ruby to complete the transformation.