Introduce ToolbarButton enum

[ericmp33]

Description

It adds the ability to use enum cases instead of hardcoded strings when working with toolbar buttons:

RichEditor::make('txt')
    ->toolbarButtons([
        [ToolbarButton::Bold, ToolbarButton::Italic, ToolbarButton::Underline],
        [ToolbarButton::AttachFiles],
    ])

Visual changes

-

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.

[danharrin]

Since custom toolbar buttons are also supported, I don't really see the value in this to be honest aside from a little autocomplete for the default buttons. For custom buttons you would still need a string, so I don't think it's worthwhile to introduce.