feat(core): make font style, weight, scale params optional

[lekhmanrus]

Change font style, weight, scale constructor parameters to optional.
Export FontStyle and FontWeight.

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• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

Creating new font instance requires redundant parameters:

new Font('material-symbols-outlined', 28, 'normal', 'normal')

What is the new behavior?

Style, weight and scale parameters are optional:

new Font('material-symbols-outlined', 28)