Lift core of "HeraldConditionResult" to "HeraldTranscriptResult"

Summary: Ref T13586. Lift the behavioral core of "HeraldConditionResult" into a new abstract base "HeraldTranscriptResult", with the intent to introduce a "HeraldRuleResult".

Test Plan:

• Ran Herald rules, reviewed transcripts.
• This change should have no behavioral effect.

Maniphest Tasks: T13586

Differential Revision: https://secure.phabricator.com/D21564