Add LanguageLevel package.

[rocky]

Add a mathics package, analygous to packages VectorAnalysis or DiscreteMathCombinatoricaV0.9`, which does what it can to provide compatibility with various WL versions.

I the beginning it could be something no more complicated than:

LanguageLevel::usage = "LanguageLevel[v] Set Mathiics to support WL Version $v$"; 
LanguageLevel[n_Integer] := Unprotect[$VersionNumber];$VersionNumber=n;Protect[$VersionNumber];

Except the above doesn't work. Later on it would include compatibility functions or changes to exisiting functions that support backwards/fowards compatibility.

See also #1131 and #1104 (comment)

[TiagoCavalcante]

It seems to be an error in the evaluation because ClearAttributes is doing what it should and when it tries to change the value of $VersionNumber the attribute Protected is still there.