Tree: ASCII_GUIDES and TREE_GUIDES promoted to class attributes

[jdvanwijk]

Type of changes

• [ ] Bug fix
• [ ] New feature
• [ x ] Documentation / docstrings
• [ ] Tests
• [ x ] Other

Checklist

• [ x ] I've run the latest black with default args on new code.
• [ x ] I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• [ ] I've added tests for new code. (New tests didn't seem necessary in this case?)
• [ x ] I accept that @willmcgugan may be pedantic in the code review.

Description

In the Tree class, I have promoted the ASCII_GUIDES and TREE_GUIDES constants (previously located within the __rich_console__ method) to class attributes. Additionally, I have updated the Tree class docstring to reflect this change.

This modification simplifies the process of customizing the appearance of tree guides. Users can now easily inherit from the Tree class and modify its class attributes directly, rather than delving into the __rich_console__ method.

[jdvanwijk]

Example use:

class WideTree(Tree):
    ASCII_GUIDES = ("        ", "|       ", "+------- ", "`------- ")
    TREE_GUIDES = [
        ("        ", "│       ", "├────── ", "└────── "),
        ("        ", "┃       ", "┣━━━━━━ ", "┗━━━━━━ "),
        ("        ", "║       ", "╠══════ ", "╚══════ "),
    ]