Add support for matching full terms at song select using suffixed !

[peppy]

Given the following beatmap artists:

• artist
• The artist name
• The artistic

You now have three methods of searching:

• artist will match all three (term can appear anywhere, even inside a word)
• "artist" will match the first two (term must be isolated in a phrase, not inside a word)
• "artist"! will only match the first (full term must match)

The ! suffix is non-standard, but feels like it will work well and also be easy for users to remember. It also has the advantage that you can add it without moving the cursor backwards in the string (which... is not currently possible at song select).

Tested to work with artist="test"! style searches too.

[bdach]

Have pushed some fixups:

• Renamed MatchMode.None to MatchMode.Substring (e3d97b3) - felt pretty weird to have a match mode of "none" that... does something
• Fixed a case wherein the .Trim('!', '\"') call could eat characters from inside the quotes (bf99fc6), with test coverage (aba380b) - maybe feels edgecasey, but a very real one to hit in specific circumstances ("circles!"!, anyone?)
• Slightly expanded test coverage to include case insensitivity (bca1a91)

Please check before merging.