feat: Add pi constant

[mark-koch]

Closes #450. Depends on #449.

• Adds a guppy.constant function to declare constants in modules
• Adds a pi constant to the angle module for convenient writing of angles

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.01493% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.94%. Comparing base (12e41e0) to head (a873151).

Files with missing lines	Patch %	Lines
guppylang/decorator.py	94.11%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #451 +/- ## ========================================== + Coverage 91.91% 91.94% +0.03% ========================================== Files 57 58 +1 Lines 5798 5849 +51 ========================================== + Hits 5329 5378 +49 - Misses 469 471 +2 ```

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