fix: support multi-segment base paths in use()

[afonsojanu]

Closes #217.

app.use('/foo/bar', handler) never actually gets hit, even for requests that should match it — it always 404s. Here's the repro from the issue:

const app = Polka();
app.use('/baz', Sirv('./static/script'));       // works: /baz/main.js
app.use('/foo/bar', Sirv('./static/script'));   // 404:   /foo/bar/main.js

The cause is in handler(): the incoming request path is truncated down to just its first "folder" (via value()) before being looked up in bwares/apps, but those tables are keyed by the full base string passed to use(). So /foo/bar/main.js gets reduced to /foo for the lookup, which never matches the /foo/bar key it was actually stored under, and the middleware/sub-app silently never runs. Single-segment bases (/baz) happen to work only because there's nothing to truncate.

This adds a small matchBase() helper that scans the registered keys and picks the longest one that's a full path-segment prefix of the request path (so /foo won't accidentally match a request for /foobar), and uses it for both the bwares and apps lookups instead of the single truncated segment. Single-segment bases keep behaving exactly as before.

Added two tests: one covering a multi-segment middleware base (plus making sure a sibling single-segment path under the same first folder isn't affected), and one covering use(base, subApp) with a multi-segment base.

Ran the full existing suite before and after with git stash — same single pre-existing failure both times (polka/url :: usage — throws if no input, unrelated to this change, looks like a Node version difference in the url.parse() deprecation path). Everything else, including the 2 new tests, passes clean.

[afonsojanu]

Closing this for now as part of a clean slate — stepping back from everything I had open across these repos.