Enable outlines for features on :focus-visible (keyboard only)

[Malvoz]

In #478 outlines were removed for both pointing device- and keyboard users because we introduced link-like styles for features (using stroke). However, because we can't (at least don't, atm) ensure sufficient contrast of strokes I think we should add back (or really not remove) outlines for keyboard users (using focus:not(:focus-visible)), because currently it can be really hard to track what is being focused. @prushforth had some concerns around this in #478 (comment).

These changes makes it easier for keyboard users to identify what they've focused, even if that means setting both stroke and outline on :focus-visible (which wouldn't be desirable in a native implementation - only outline would be visible/change on focus in a standardized map viewer, so unless there's a way to "wrap" outlines to SVG shapes I think we have to go with this for now).

Current styles on master branch:

feature-current--focus-styles.mp4

With these changes we're now showing outlines on keyboard focus:

focus-visible-path-stroke-none.mp4

Although not all focusable paths show an outline, had to remove if(layer.outlinePath) p.path.style.stroke = "none", and add a very generic path[tabindex="0"]:focus selector to the feature focus styles.

Final result:

focus-visible-path-stroke-none-removed.mp4

[prushforth]

@Malvoz this commit reveals an untested regression. If you look at this page, it shows the tile boundaries, which it shouldn't. I'll open an issue for the regression, but can you take a look at a fix for this? Thanks!