Add method to check whether WorkspaceDefaultMembers has a value

[foresterre]

This method checks whether the called cargo-metadata supports reading the workspace default members, which is false for versions of Cargo prior to 1.71

Alternatively, I could:

• add an Option variant method like workspace_default_packages directly on Metadata: pub fn workspace_default_packages(&self) -> Option<Vec<&Package>>, or
• reverse the name from is_available to is_missing (which would also allow the removal of the doc(hidden) function workspace_default_members_is_missing).

fixes #254

[oli-obk]

• reverse the name from is_available to is_missing (which would also allow the removal of the doc(hidden) function workspace_default_members_is_missing).

is this test usually going to be used as if !foo.is_available() {...} or as if foo.is_available() {}? Oh to be honest we could just add both, just like Option has is_none and is_some.

[foresterre]

I've updated the PR to include both.

I've also removed the doc(hidden) workspace_default_members_is_missing, which is no longer necessary when the is_missing variant exists. It was both marked hidden and noted that it "might be removed in the future". Are you okay with its removal?