Implement rendering attributes

[naveen521kk]

• The attributes passed to Layout are first converted to Pango understandable attributes. This is done in attributes.pxi file.
• The attributes are then applied to the PangoLayout when creating the Layout object.
• Both SVG renderer and PNG renderer support the attributes.
• Move the convert_hex_to_rgb function to utils.

Here's a small example to use this feature:

import manimpango

# match the characters from index 0 to 5.
attribute = manimpango.TextAttribute(0, 5)
attribute.family = "Cursive" # set to a cursive font
attribute.background_color = "#123456" # change background_color

new_attribute = manimpango.TextAttribute(5, 10) # characters 5 to 10
new_attribute.family = "Algerian" # font

layout = manimpango.Layout(
    "hello world",
    attributes=[attribute, new_attribute],
    font_desc=manimpango.FontDescription(
        family="Serif",
        size=30,
    ),
)
renderer = manimpango.PNGRenderer("test.png", 1000, 100, layout)
renderer.render()

This should render the following image (test.png):

Lots of other attributes which Pango provides aren't implemented yet, needs some work there.

[naveen521kk]

I'll merge this once I get time to write some tests

[PhilippImhof]

This looks very nice. Looking forward to you finishing that. 👍