Parse features with `+` in their name
    
    If a crate has a feature with `+` in it's name,
    the `%__cargo_feature_from_name` macro fails to correctly parse it.
    For example, the `cxxbridge-flags` crate declares (among others)
    the `c++20` feature; that is currently parsed as follows:
    
    ```lua
    > string.match("cxxbridge-flags+c++20-devel", "^.+%+(.+)-devel$")
    20
    ```
    
    The adjusted regex matches the *first* `+` as feature name separator:
    
    ```lua
    > string.match("cxxbridge-flags+c++20-devel", "^[^+]+%+(.+)-devel$")
    c++20
    ```