[feaLib] Parse `languagesystem` in the same way as all other statements
[feaLib] Parse `languagesystem` in the same way as all other statements

In the new version, the parse_languagesystem_() function returns
an ast.LanguageSystemStatement, which then gets appended to the
current list of statements by the caller. Before this change,
the parse_languagesystem_() function would always append the
newly parsed statement to the list of top-level statements.
In practice, it does not matter because `languagesystem` is
only valid at the top level, but there is no good reason why
the parser method would have to be different from all other
constructs.

googlefonts/fonttools,fonttools/fonttools