Allow configuration of default fonts used in TextView

[SergioEstevao]

This PR brings to TextView the following:

• The possibility to replace the defaultFont being used after the TextView is created
• Introduction of a FontProvider to allow configuration of fonts being used in the system
• Configuration of the default font being used for monospace

To test:

• Run the sample app
• Check the demo content
• There is new button on the top right (+) press it and see if the default font changes in all the demo content.

[etoledom]

Thank you @SergioEstevao for this Improvement!

I tested as described and it works well.
But I noticed that the font changes in all the content that doesn't have any styling. Bold, Italic, heading, etc keeps its current font.

I did a test with a more obvious font:

Is this expected?

This is Zapfino font. But it doesn't have Bold or Italic variant.
Also tested with Optima font that does have Bold and Italic variant and the result was the same.

One more thing: When I tested with Zapfino I got a crash here:

AztecEditor-iOS/Aztec/Classes/TextKit/LayoutManager.swift

Line 321 in f9ff2d3

return UIFont(descriptor: descriptor!, size: font.pointSize)

It was quick fix by replacing the force unwrap with an if-let. But does it make sense that the will be nil given that font and traits are not nil? 😬

[SergioEstevao]

@etoledom code updated ready for another look.

[etoledom]

Thank you for the update @SergioEstevao !
It's working great now 🎉

I left a code comment to think about, but everything looks great, so ✅