feat(commands): implement :CP open [problem|contest|standings] (#319)

## Problem

There was no way to open a problem, contest, or standings page in the
browser from within the plugin.

## Solution

Add `contest_url` and `standings_url` to `MetadataResult` and persist
them in the cache. Add `cache.get_open_urls` to resolve all three URLs.
Wire up `:CP open [problem|contest|standings]` in `commands/init.lua`
to call `vim.ui.open`, warning when a URL is unavailable (e.g. CSES
has no standings). Closes #315.

scrapers/cses.py

@@ -218,6 +218,8 @@ class CSESScraper(BaseScraper):
             contest_id=contest_id,
             problems=problems,
             url="https://cses.fi/problemset/task/%s",
+            contest_url="https://cses.fi/problemset",
+            standings_url="",
         )
 
     async def scrape_contest_list(self) -> ContestListResult: