PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/DevelopmentWorkflow.md#running-unit-tests.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/WritingUnitTests.md.

What is the current behavior?

NativeScript does not support text-shadow. This has been a requested feature (#3597, #5268).

What is the new behavior?

Adds text-shadow css property (and textShadow attribute).

It implements a subset of the text-shadow web syntax. The expected format is:

<offset-x> <offset-y> <blur-radius> <color>
e.g.
2 10 4 rgb(255, 100, 100)
2px 10px 2px rgba(10, 10, 10, 0.5)
1 1 1 #55a
2 2 2 #aaa

The reasons for this limitation are:

• some of the default values in the web syntax are not clear (i.e., people shouldn't use them anyway);
• It is impossible to implement multiple shadows (e.g. 1px 1px 2px red, 0 0 1em blue, 0 0 0.2em blue;);
• parsing/tokenisation would not be trivial because there are multiple formats for colors (rba(0, 0, 0), #fff, etc.). It would be a lot of work;

I thought I should have more feedback on this PR before adding more work.

I think it PR would still need:

• Unit tests (I didn't add them yet, but I added a demo for this feature in the apps.ui);
• Add more syntax options (maybe? which ones?);
• Validation/syntax errors;
• Support for formatted strings and spans (not sure if it's even possible);
• Support for tab headers (not sure if it's even possible);

Demo (apps.ui):

Fixes/Implements/Closes #[Issue Number].
#3597 and part of #5268.