Simplify <Link> active semantics

[taion]

Prerequisites: #3158, #3172

Once we merge #3158, we'll be able to dramatically simplify the active checking for index-only links – that check will just be a comparison of the link to the path name, with a special case for handling trailing slashes (as the pathname will be canonical outside of the trailing slash).

For consistency, we should consider also updating the semantics on non-index links to just do a prefix check, once we also resolve #3172. Given issues like #3230 (and many related ones), users seem to be confused with the current behavior of path active status calculation. Also, this will dramatically improve #1873, as it will let us avoid running a full route match just to determine if a link path is active.

[taion]

(but contra what I said in #1873 (comment), in the interest of not making breaking changes without extremely good cause, we should not make onlyActiveOnIndex be the default)

[timdorr]

@taion Did you want to tackle the #3158-related issue? I just merged that in.

[taion]

I want to hold off until we get a release cut. I really don't like having a huge stack of unreleased changes sitting on master.

[taion]

More people who are confused by the current behavior:

• Keeping the active link for child routes. #3237
• isActive does not work for optional params #3277

[taion]

I chatted with @ryanflorence on this. I'm convinced now that the current semantics for route active status are better, because they're more powerful than prefix checking. I'll re-open #3277 and separately just optimize index link handling for now.