This commit moves _getMultilineContents and _applySurround into
TextareaTextManipulation, so other text area components using
that mixin can benefit from them (such as the chat composer).
It also creates a public function wrapper for many TextareaTextManipulation
functions that should not have underscore prefixes because they are
used outside the file. Will make follow-up PRs for each plugin/theme using
those functions then a final follow-up core PR to fix these up.