Add SoftLineBreakSyntax

[chenzhiguang]

To strictly improve the test coverage

[coveralls]

Pull Request Test Coverage Report for Build 3352126952

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 4 of 4 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 94.992%

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Totals
Change from base Build 3352023435:	0.02%
Covered Lines:	1214
Relevant Lines:	1278

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💛 - Coveralls

[kevmoo]

@chenzhiguang – please rebase on master – we now have flutter_markdown validation in place

If there are breaks there, we'll have to dig to see what's expected/intended.