Added a more suitable grammar for Mercury

Changing the grammar used for highlighting Mercury from Prolog to Mercury,
since Mercury defines many additional syntactic constructs.
A [lightshow demonstration](https://lightshow.githubapp.com/?utf8=%E2%9C%93&scope=from-url&grammar_url=https%3A%2F%2Fraw.githubusercontent.com%2Fsebgod%2Fmercury-tmlanguage%2Fmaster%2FMercury.tmLanguage&grammar_text=&code_source=from-url&code_url=https%3A%2F%2Fraw.githubusercontent.com%2FMercury-Language%2Fmercury%2Fmaster%2Flibrary%2Fsparse_bitset.m&code=) examplifies this.