Show a text diff in the exception message for a text mismatch

[SimonCropp]

When a text snapshot doesn't match and no string comparer returned a message, the exception now shows a line diff instead of both texts in full. The diff comes from DiffEngine's new TextDiff.Format (VerifyTests/DiffEngine#909, released in 20.6.0). This gives the equivalent of Verify.DiffPlex without installing it.

Received: Tests.Foo.received.txt
Verified: Tests.Foo.verified.txt
Diff:
2 line 2
- line 3
+ line three
4 line 4

Settings

• VerifierSettings.UseTextDiffFormat(TextDiffFormat): Compact (the default), Full or Minimal. These match Verify.DiffPlex's OutputType layouts.
• VerifierSettings.DisableTextDiff(): show both texts in full, as before.

Behaviour

• Where the diff is built. It's in VerifyExceptionMessageBuilder, not in a default string comparer, so it doesn't change how equality is decided.
• Comparers still win. A comparer's message still takes precedence, so existing Verify.DiffPlex users see no change until they remove it.
• Exact comparison. Lines compare exactly, whitespace and case included. Verify.DiffPlex ignored whitespace, so a failure caused only by whitespace showed no changed lines.
• Fallback. Texts that differ only in line endings produce an empty diff, and the message falls back to both texts in full.
• Inline snapshots are unchanged. They still show received and expected in full.

Other changes

• DiffEngine 20.5.2 → 20.6.0.
• Docs: a new "Text Diff Format" section in exception-message-format. The verify-options text and the Verify.DiffPlex entry in comparer are updated.

Tests

• New VerifyExceptionMessageBuilderTests cover each format, the diff disabled, a whitespace-only change, and the line-endings fallback.
• Eight existing message snapshots changed. The only difference in each is that the full text became a Diff: block.
• Verify.ExceptionParsing.Tests pass on net11.0 and net48, and Verify.Tests pass on net11.0.