Icon themes could only map files by exact name or extension and folders by name. Themes can now match by regular expression or path (patterns that could backtrack badly, lookaround, inline flags and named groups are refused at install), ship dark variants, replace built-in UI and symbol (PSI) icons, and offer packs (groups) and variants (choices). A theme describes its own options page in Settings › Plugins as sections of items naming options the IDE implements (hide file/folder/UI/PSI icons, hollow folders, accent, monochrome, saturation, opacity, icon size, row height); values are stored per theme in appearance.fileIconThemeOptions. Existing themes are unchanged.
Closes#1096
Co-authored-by: pdasilem <74730932+pdasilem@users.noreply.github.com>
