ci: check `metadata.toml` files

The prob-specs [`ci.yml`](https://github.com/exercism/problem-specifications/commit/514d1dbb549dde4712d38f2c230604c0bbc39141) workflow checks Markdown and JSON files, but not the `metadata.toml` files.

In the hopefully-unlikely event that a strictly invalid `metadata.toml` file reaches `main`, `configlet sync` would produce an error to complain about the invalid TOML when the syncing scope includes metadata.

We could check that:

- every exercise has a `metadata.toml` file
- with a valid `title` key/value pair
- with a valid `blurb` key/value pair
- if the `source` key is present, the value is valid
- if the `source_url` key is present, the value is valid
- there is no other key/value pair (it may be a typo of `source` or `source_url`, like `source-url`)