[fea-rs] Accept valueRecordDef inside feature and lookup blocks

[anthrotype]

The FEA spec says valueRecordDef is a top-level-only statement, but feaLib has accepted it inside feature and lookup blocks since valueRecordDef was first implemented (fonttools/fonttools@c06a377aa, Aug 2015), it was never limited to top-level scope. And as we found out today, real fonts rely on this: e.g. Google Sans Arabic defines a named value record inside a feature block:

feature kern {
    # ...
    valueRecordDef <0 -260 0 0> DOT_SHIFT;
    lookup yehbarree_marks_arabic { ... } yehbarree_marks_arabic;
    # ...
} kern;

This adds valueRecordDef handling to the parser, validator, and compiler for feature/lookup block scope.

Part of #2014.