feat(core): add new text-shadow property

[tralves]

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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.

[tralves]

Hi!
I see that this is on the roadmap :D. As I said this PR needs some polish. Do you have pointers or tips to what I should do to make your lives easier?

[farfromrefug]

Why did you remove that comment ? It is actually an important comment

[farfromrefug]

First I dont think we need a default here. We have to be careful. Get default are slow because they make native calls. And yours even make 4 of them.
We now a default view as no shadow so not having a default value should be fine and faster.
Also you should use a const value to access native view protected instead of accessing it 4 times. If a class define it as a getter your would unnecessarly call it 4 times

[farfromrefug]

Why create a _setShadow method If you call it only once?
It makes the class."bigger" for now reason

[farfromrefug]

@NathanWalker you merged this without review comments being fixed ? that PR removes things which should not have been touched here !

[rigor789]

@farfromrefug it's on the release/8.0.0 branch where we are still going to do cleanups & changes. Just helps manage things together - and mainly helps resolving any conflicts early.

[farfromrefug]

I get it. just that PR removes comments which should remain there for everyone to understand things.
I don't see how you are going to bring them back now (except manually obviously) now that it is merged.
thanks for your work guys !