Add selectrum-default-value-format customizable variable

[minad]

• This variable allows the user to customize the format
  of the default value indicator
• The variable can also be nil in order to hide the indicator
• Do not remove the properties from the candidate string,
  for example Consult uses candidate strings with the invisible
  property
• See also Prompt shows [default value: ...] for Consult narrowing #444

[minad]

For simplicity I went with the format string approach for now. This is also more aligned with selectrum-multiline-display-settings which also uses a format string.

I find the current candidate highlighting a bit questionable. Maybe it would be okay to always use the minibuffer-prompt face?

                          (if (and selectrum--current-candidate-index
                                   (< selectrum--current-candidate-index
                                      0))
                              'selectrum-current-candidate
                            'minibuffer-prompt)))))

[clemera]

Great, thanks!

[minad]

What about this question:

I find the current candidate highlighting a bit questionable. Maybe it would be okay to always use the minibuffer-prompt face?

One more question - I am not sure if nil is part of the defcustom string type spec. Sometimes nil is coerced to a string, sometimes not. So it could also be (choice (const nil) string) (FIXED, should be choice).

[clemera]

I find the current candidate highlighting a bit questionable. Maybe it would be okay to always use the minibuffer-prompt face?

I don't know I don't think I would like that, using the same face as for candidates makes it look like a usual candidate which I like.

One more question - I am not sure if nil is part of the defcustom string type spec. Sometimes nil is coerced to a string, sometimes not. So it could also be (choice (const nil) string).

I'm not picky about it but if its not part of the spec than why not use (choice (const nil) string)?

[minad]

I don't know I don't think I would like that, using the same face as for candidates makes it look like a usual candidate which I like.

Okay, so keep it as is? Then I am finished with the PR - ready to merge.

[clemera]

Thanks!