Add new filter to asset version

[JPry]

This PR adds a new filter: stellarwp/assets/{$hook_prefix}/version. This allows filtering the asset version string when it does not come from an asset file.

This is particularly useful in development when the asset file has changes but the $version value used when registering the asset hasn't changed. E.g. for an asset jpry-cool-asset, I could filter the version to something like time() in this way:

$prefix = 'jpry';

add_filter(
	"stellarwp/assets/{$prefix}/version",
	function ( $version, $slug ) {
		return 'jpry-cool-asset' === $slug
			? time()
			: $version;
	},
	10,
	2
);

[borkweb]

This is a great addition! We can remove the since line from the docblock for the library, though.

[JPry]

@borkweb Thanks! I removed the @since tag.