[flutter_markdown]: Invert order of style merging

[domesticmouse]

Fixes: flutter/flutter#81720

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[flutter-dashboard]

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[csells]

@navaronbracke: I noticed what a great job you did on flutter/flutter#158428. would you be able to take a look at this one? the recommended fix is in place, but the new test results need to be verified and the tests updated for the new blockquote styling behavor.

[navaronbracke]

@csells If I were to look at this PR, what would be expected of me here? The PR doesn't yet link to an issue that it fixes, so I'm unsure what the expected behavior is. Do you need me to write a test for this PR?

what a great job you did on flutter/flutter#158428

That was only a very small fix, though :p

[csells]

@navaronbracke: actually, I expect this fix to be even smaller. here's the bug report: flutter/flutter#81720