fix(firestore): Export Filter type from Firestore

[jhuleatt]

Discussion

Fixes #2191, missing export for Filter from Firestore. This builds off of #2167 (and includes @danReynolds as commit co-author) by addressing #2167 (comment).

Testing

Added a test to ensure Filter is exported

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[lahirumaramba]

Thank you @jhuleatt and @danReynolds!
LGTM!