Added the length property to the String class (v3.1.10)

[simon-ritchie]

Since v3.1.10, the length property has been added to the String class.

import apysc as ap

string: ap.String = ap.String("ABCDEF")
length: ap.Int = string.length
ap.assert_equal(length, 6)

For more details, please see the following document:

• String class length property