feat!: make indicator affixes configurable

[bdarcus]

The current indicators are hard-coded, with the only configurable details being the symbols in citar-symbols, and the space between them in citar-symbol-separator.

This makes all of it configurable by adding:

• citar-indicators variable; a list of plist indicator specs
• citar--make-indicator-processors function (creates a list of plists with the needed properties, including the compiled predicate functions)
• citar--make-indicators-tags and citar--make-indicator-symbols functions
• add indicator variables, and a new wiki page with others

BREAKING CHANGE: citar-symbols will be deprecated in favor of citar-indicators

Close: #601

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Status

From what I can tell, everything works correctly; this example removes the links indicator, and uses default for files, and icons for notes:

And while I haven't benchmarked, I don't believe this should be any slower.

Aside: I had an idea to extend this later so people can separately configure left (what we do now) and right sides of the affixation.

I think I may turn citar-indicators into a defcustom, but later.

In the second commit I switched to using a cl-defstruct for the citar-indicator type. Maybe a tad overkill, but it mirrors what we do with citar-cache, and it provides some advantages from a coding POV. I've made this function public to encourage people to use it in their configs.

@andersjohansson with some help, I figured out the code for this. When I merge this, you could use it for #744.

[bdarcus]

OK, merged. I think it's pretty solid, though let me know, or send a PR, if any little bugs.

Does occur to me I didn't really include error handling in the key functions, so that's a good next step.