Add config option to configure font fallback

[Yaraslaut]

Refs #225
Refs #1374
PR adds configuration option to disable font fallback

Implemented following logic: fallback entry in the config has several options: auto none or a list of fonts [ "A" , "B" ]
auto will insert all fonts in fallback list
none will make fallback list empty
list will put in fallback list fonts in the same order as they appear in configuration given that they are found

[christianparpart]

Hmmm 🤔
Is this really useful, to be able to completely turn off font fallback?

Even if, we should not do something like if (useFontFallback) ... else ... in the rendering hot path, but rely on function pointers / virtual dispatch instead. I know this sounds like a micro optimization, but would be cleaner IMHO on top of it, too.

Ignoring that, I think we could instead promote the mock font locator instead, because there you can explicitly declare how many fonts (font files) to pick in order for fallback. If one would not specify any more than a single font, that would be the same as having no font fallback at all.

But none of it is actually addressing the point in #225.

We could instead work on embedding fonts (Nerd Fonts and standard emoji fonts), which would help people like @eproxus to not require font patching (nerd font), or always come with the same experience when enabling something like use_builtin_emoji: true (or emoji: builtin or not specifying emoji: key at all, indicating that the builtin should be used)

[Yaraslaut]

I updated PR, now we configure fallback on the stage of font_locator and also can specify list of fonts that use for fallback

[christianparpart]

Thank you, @Yaraslaut.

In the long run, we should try to also extend the documentation as we go with the code changes. This is a user-facing configuration change, which would fit nicely into our website's documentation as well :)