Add a better `parse` return type according to provided `value`

Hey 👋

Currently return type of `parse` is `number | undefined`. It would be nice if `value` was checked via TS types and return just `number` when `value` is for example a valid `60s`.

@lukeed/ms - v2.0.2

Start at the parse API and its current type declarations, then inspect how the value argument is represented in the package. Define the expected narrowing for valid values such as `60s`, while preserving `undefined` when validity cannot be established, and verify the public type behavior with the project's existing checks.

Written by the indexing model from the issue text.