GDScript DocGen: Fix and improve appearance of metatypes and values

[dalexeev]

1. Fix metatypes are not taken into account in DocGen.
2. Fix BBCode markup has effect in EditorHelp.
3. Fix special characters other than \n not appearing in strings.
4. Fix unwanted \ns when displaying dictionaries.
5. Don't display serialized objects.
6. Add support for displaying typed arrays in constants.
7. Don't display deeply nested arrays/dictionaries. Perhaps this is not a good heuristic, it’s just that in this implementation you need to specify some kind of limit, and I decided that 100 was too much. In the future, we may improve content clipping when the length exceeding some limit.

Test script

class MyClass: pass
enum MyEnum {A, B, C}

const BROKEN_MARKUP = "test [int] test"
const NESTED_ARRAYS = [1, [2, [3, [4]]]]
const TYPED_ARRAY: Array[int] = [1, 2, 3]

var native_metatype := Node
var class_metatype := MyClass
var enum_metatype := MyEnum
var special_chars := "aaa\nbbb\tccc"

Before

After

[adamscott]

Superfluous comment here

[adamscott]

Looks fine! The code is very clean, thanks Danil!

[akien-mga]

Thanks!