link the zigBuiltinFn group to Function instead of Statement by default

Previously builtin functions were the only thing mapped to "Statement".
Now, some control flow keywords are mapped to "Statement", so builtin
functions are instead mapped to the more reasonable "Function" group.

I believe that "Statement" was previously picked to ensure that builtin
functions get a color that would stand out and be unique. Function may
not get an equally prominent color but still be unique (usually).

Depending on how this affects various colorschemes, it may also make
sense to map builtin functions to a different group like "Special".

Author: Techatrix

syntax/zig.vim

@@ -66,7 +66,7 @@ highlight default link zigHexNumber zigNumber
 highlight default link zigOctNumber zigNumber
 highlight default link zigBinNumber zigNumber
 
-highlight default link zigBuiltinFn Statement
+highlight default link zigBuiltinFn Function
 highlight default link zigKeyword Keyword
 highlight default link zigType Type
 highlight default link zigCommentLine Comment