Change scope-name to scope.lilypond

[Alhadis]

In TextMate terminology, a grammar's scopeName is equivalent to a globally-unique identifier. There's no reason this should be named anything other than source.lilypond, which falls in line with every other LilyPond grammar.

[Alhadis]

@yrammos It'd be great to get this merged and released for GitHub Linguist. Currently, my roff-language package has been forced to include two different scopeNames:to highlight regions of embedded LilyPond grammar:

• AtLilyPond ~ Which is the scopeName used by Atom usersh who install this
• However, there's a different LilyPond grammar with a scopeName of simply source.lilypond (as per the original TextMate package's choice of scope.. And this happens to be the package that provoides LilyPond grammar highlighting on GitHub.

I'd prefer to use only one scopeName so this error is silenced and we can help make the world a simpler place. ;)

[yrammos]

@Alhadis makes sense, thanks for the PR.