wip: adds $nav-link-* vars

[zalog]

This PR adds a new vars for $nav-link-*'s:

$nav-link-font-size:                null !default;
$nav-link-font-style:               null !default;
$nav-link-font-weight:              null !default;
$nav-link-color:                    $primary !default;
$nav-link-hover-color:              darken($primary, 15%) !default;

This PR depends on #30622

[zalog]

I think we also need to add hover and active states as well. What do you think?

[MartijnCuppens]

I think we also need to add hover and active states as well. What do you think?

Any suggestions theming wise?

[zalog]

I'll come back here soon, this days.

[zalog]

Removed wip and rebased.

[ffoodd]

LGTM.

FWIW, I'd go with the active state too — even if the active state should not rely on color only (but this is another topic to tackle in Bootstrap and/or examples, I guess).

@MartijnCuppens Do you agree, about an active state variable?

[mdo]

Closing for #31035.