This provides an option to override the functionality provided by the
theme, or apply `text-transform` outside of the theme.
The functionality of the `text-transform` is as below:
- `text-transform: none` - will disable **any** transformation (like the uppercasing by `terminal` theme)
- `text-transform: uppercase` (uppercase not upper as per your message) - will force all characters into uppercase.
- `text-transform: lowercase` - will force all characters into lowercase.
- `text-transform: capitalize` - will uppercase the first letter of every word
In addition, this commit introduces:
- helper methods on the `d2graph.Style` struct to determine the type of
`text-transform` to be applied.
- `ApplyTextTransform` method on the `d2graph.Attributes` which will
transform the `Label.Value` to the correct text case.
