track language changes, improve attr handling in GtkSourceView language-spec

Refactors parsing of numerical literals to make it more readable.
Removes 'float'/the 'f' literal suffix and invalid character literals ''' and '\'.
Also makes attribute highlighting more robust and allows urls in attributes to be recognized.
auto merge of #9921 : sp3d/rust/master, r=alexcrichton

Refactors parsing of numerical literals to make it more readable.
Removes 'float'/the 'f' literal suffix and invalid character literals ''' and '\'.
Also makes attribute highlighting more robust and allows urls in attributes to be recognized.