Add option to auto navigate to the first item on open

[anleac]

This is an extension to the firstDefaultOption prop, and infact, a replacement, but extending an additional mode to this to now allow for the following three behaviours as a config prop:

• Do nothing on open.
• Auto default to the first option, but not have it selected (this is what would happen currently if firstDefaultOption is true).
• Auto navigate to the first option on open (if there is an appropriate option), this is the new behaviour.

This PR does the following:

• Replaces the firstDefaultOption boolean with a type config of three options
• Adds some basic tests

More than happy to update the naming if people feel strongly here.

[iansan5653]

Would it work to add an else if (this.firstOptionSelectionMode === 'focused') {this.navigate(1)} case here instead of defining a separate focusDefaultOptionIfNeeded method? That way we only have to call one function in start and we can't forget to add the second case if we call indicateDefaultOption elsewhere.

[anleac]

I opt'ed for a new method because this method was also being called here:

combobox-nav/src/index.ts

Line 136 in c28985b

this.indicateDefaultOption()

- where we don't want to call navigate on. Wdyt?

[iansan5653]

ooh I see. I think I still would rather combine the methods, but then maybe here we add a conditional if (this.firstOptionSelectionMode === 'active') (or if (this.firstOptionSelectionMode !== 'selected')?)

[anleac]

Agreed! Addressed here: 1ef750e

[iansan5653]

Looks great! Just a documentation suggestion

[iansan5653]

Oh! We should also update the docs in the README - I just realized they describe the settings as well. We could probably use the same text as in the proposed comment.