Place a space between certain character class letters only

[nobu]

Basis

Japanese text is not usually space-separated per words, and can be folded at almost any places except for around some punctuations.
That means, line breaks in Japanese sentences should be removed and not replaced with spaces like in English sentences.

Problem

However, RDoc blindly replaces line breaks with spaces, resulting in very unnatural gaps in Japanese text, highlighted in the screenshot below.

This makes it impossible to select words by double-clicking, for example.

Fix

With this patch, only line breaks between SPACE_SEPARATED_LETTER_CLASS characters will be replaced with spaces.

SPACE_SEPARATED_LETTER_CLASS includes Decimal_Number, Cased_Letter, and Connector_Punctuation for now, as an analog to \w.