text::Shaping strategy selection

[hecrj]

This PR introduces a new Shaping enum in the text module that allows users to control the shaping strategy used to lay out text.

The Basic strategy is considerably more efficient (specially in debug builds!) and works well for simple scripts (the use cases the library currently satisfies).

Because of this, Basic is the default strategy used in text-based widgets (except the TextInput). In other words, Advanced shaping must be explicitly enabled when necessary.

[AustinMReppert]

As some one who knows nothing about fonts, I feel like this could result in some problems:

• Looks fine in dev/test, but user enters text requiring advanced shaping and app re-displays said text incorrectly
• App dev localizes their app's text and ships something that appears fine to them.

I wonder if there is any room for warnings in debug mode to detect text that requires advanced shaping.

[hecrj]

We will get there eventually.

Developers need to know what they are doing for now.

[MoSal]

Would making the default cfg-able with a default_advanced_shaping feature be a bad idea?

Also, wouldn't it be better if the enum was marked as #[non_exhaustive], with the assumption that Auto or some other shaping impl/strategy may get implemented in the future?

[pomoke]

Simply make advanced shaping default, and this is how a lot of GUI frameworks work. i18n will heavily depend on fallback and shaping.

[hecrj]

Eventually, maybe.