Fix an issue with tail parsing in object embeds in remarkup

Summary:
Fixes T6619. In {Xnnn key=value, key=value} we did not require a separator between the object and the key-value part. This could lead to {rX11aaa} being parsed as {rX11 aaa}, i.e. a reference to rX11 with parameter aaa set.

Instead, require a space or comma before we'll parse key-value parts of embedded objects.

Test Plan:
Added and executed unit tests.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6619

Differential Revision: https://secure.phabricator.com/D10915