Missing math constants `THIRD_PI`, `DEG_TO_RAD`, `RAD_TO_DEG`

[SableRaf]

Following up on #138 which added PI, TWO_PI, HALF_PI, QUARTER_PI, TAU, Processing also sets the following math constants:

• THIRD_PI
• DEG_TO_RAD
• RAD_TO_DEG

I wasn't aware of these when I wrote #128, so shoutout to GoToLoop for bringing this up on the forum.

Side note: the full list of Processing's constants can be found in the Javadoc:

• Documentation: https://processing.github.io/processing4-javadocs/processing/core/PConstants.html
• Values: https://processing.github.io/processing4-javadocs/constant-values.html#processing.core.PConstants