Fix LT-21934: fork task to stop parser so control returns to user

[jtmaxwell3]

This is about https://jira.sil.org/browse/LT-21934.

Michael Lee asked that StopParser could return immediately so the user can get back to work while the parser still times out gracefully in the background. I added a condition that thread.Stop() wouldn't be called while the parser was updating wordforms since thread.Stop() turns into an abort after 60 seconds, and that can leave things in a funny state.

Jason wanted to review this code.

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This change is 

[jasonleenaylor]

Reviewed 5 of 5 files at r1, 1 of 2 files at r2, all commit messages.
Reviewable status: 4 of 5 files reviewed, all discussions resolved (waiting on @jtmaxwell3)

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Src/LexText/ParserCore/ParseFiler.cs line 88 at r2 (raw file):

		/// Are we updating word forms?
		/// </summary>
		public bool UpdatingWordforms;

No longer needed right?

[jasonleenaylor]

Reviewed 1 of 1 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jtmaxwell3)