Feature add support text-shadow style

[ngthailam]

Add support for https://github.com/daohoangson/flutter_widget_from_html/issues/586

The structure is kinda new to me, so please take a look and feel free to drop comments, i'll try to address them. I follow the examples + rules here https://developer.mozilla.org/en-US/docs/Web/CSS/text-shadow

Changes:

• Add basic support for text-shadow
• Add Text Shadow showcase in Demo App

[daohoangson]

This looks great already. We need tests to cover different combinations of the shadow I think.

[ngthailam]

@daohoangson Cool if the code looks good, I'll start on writing the tests. Will update you after done

[daohoangson]

Thank you.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (dcdada3) 99.58% compared to head (98f4c2a) 99.58%. Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1158 +/- ## ======================================= Coverage 99.58% 99.58% ======================================= Files 72 74 +2 Lines 4335 4379 +44 ======================================= + Hits 4317 4361 +44 Misses 18 18 ```

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[daohoangson]

Please ignore the flaky iOS integration test, I'm working on that at #1141.

There are only 2 uncovered lines left, can you take a look?

I think adding a test case with multiple shadows should be enough.

[ngthailam]

@daohoangson I have added the extra test case

[ngthailam]

I have added the changes according to the review, please have a look