Reformat `Dialogue` syntax

[nndda]

Currently Dialogue 'raw' text file use 4 space indentation to differentiate between character name, dialogue body, and function calls:

character_name
    "dialogue body goes here"
    func()

~Godot can use either tab or spaces for indentation. To avoid any conflict I think its better to use symbols/characters instead. If the whitespaces are trimmed, this also makes indentation optional.~

Maybe its better to determine wether the editor is using spaces or tabs using text_editor/behavior/indent/type

Constructing the variable with a direct string input, within a function in a script, also count the function's indents.

func _ready():
    var dlg = Dialogue.new("""
    foo:
       Hello, World!
     """)

Resulted as:

    foo:
       Hello, World!