Also add ';' to some langaugesystem test cases. This makes the
snippets syntactically valid. The parser is still expected to
reject them for other reasons, just as before this change.
This simplifies the public API to the library. For clients, it does
not matter which exact component was detecting an error. And we will
soon have more components; there would be little point in declaring
CompilerError, TableBuilderError, and so forth.
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.
We will soon support additional blocks beyond `feature`,
and keeping this refactoring separate from new functionality
makes it easier for code reviewers to follow the changes.
