feat: add markup support for labels

[Axenide]

Instead of having to do set_markup() for every label, now you have the markup property which is a boolean.
By default, the label is plain text. With markup = True the label will now accept markup.

For example:

This code:

        self.run_label = Label(
            name="run-label",
            label="<span>&#xec2c;</span>",
        )

Will give this result:

With this addition, this code:

        self.run_label = Label(
            name="run-label",
            label="<span>&#xec2c;</span>",
            markup=True,
        )

Will give this result:

[its-darsh]

LGTM