Add individual heading weight to MantineProvider

[StevenStavrakis]

Currently, MantineProvider only tracks fontSize and lineHeight for headings. Designers often use weight to distinguish between headings, and control over the individual weight from the theme object should be considered a basic feature. While this can be handled in global CSS, I can't find a good reason why it should be. Putting heading weight controls in the theme would decrease the splitting of styles into multiple locations and allow for better control of type.

[rtivital]

We will consider adding this feature in one of 4.x releases.