On Linux, use CJK fallback fonts for Tertiary Ideographic Plane

[delan]

When a character isn’t supported by any of the fonts chosen by the page, some browsers search through all of the installed fonts to find one that supports text in that language (e.g. Firefox). Servo can’t do that yet, but we currently have hardcoded lists of “fallback fonts” that we try in those situations. There are separate lists for each platform, and on Linux, we use the list in freetype/font_list.rs.

For text in Chinese, Japanese, or Korean, we add an extra list of fallback fonts, but our check for those languages is outdated. Update the check to include characters in the new Tertiary Ideographic Plane (plane 3), which is entirely dedicated to CJK languages.

For testing, you can use the character 𰻝 (U+30EDD), or the following test page:
data:text/html;charset=utf-8,%F0%B0%BB%9D