Treat punctuation as words and as word separators

[kitbdev]

• fixes Inconsistent editor movement near arithmetic operators #92981

Treat punctuation as words and word separators.
This affects behavior in TextEdit, LineEdit, and RichTextLabel for word selection and operations.

Changed a test for ui_text_backspace_word since a period after a word is now separate and won't be removed together. This matches the behavior of other editors.

• Added simple test for custom word separators from Enable custom separators to treat different characters as words #92514.
• Added tests to prevent issues like Inconsistent and Broken Skipping Cursor in Editor - Godot 4.3 Beta 2 Custom #93618, Double clicking on a word can select prior not character symbol #69254, or Double clicking CJK text does not select the word under the cursor #75808.