Support ligatures for fallback fonts

[driverkt]

It would be nice if ligatures could be specified on a per-font basis to better support font-fallback.

For example, say I have a font (ex: MonoLisa) which has several ligature options, but it is only installed on machines with macOS. If I don't have this font installed on a Linux host, I may want to fall back to another font (ex: Monaspace Argon).

With editor.fontFamily font fallback is possible, but there is only one "set" of values allowed in editor.fontLigatures. The ligatures I have specified for my primary font may not correspond to the ligatures I wish to specify for my fallback font.

Please let me know if any clarification is needed.