Add more `Line` attributes

[itzpr3d4t0r]

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Adds the following Line attributes:

• center
• centerx/y
• angle
• slope

Credits

Geometry Project: For code, docs and tests: @novialriptide @Emc2356 @itzpr3d4t0r @ScriptLineStudios @avaxar @Matiiss @newpaxonian @maqa41 @blankRiot96 Also thanks to @Starbuck5 for kickstarting the idea and the occasional help!

Functionality added in this PR

• geometry.pyi https://github.com/pygame-community/pygame-geometry/blame/main/geometry.pyi Credits to @Emc2356 @itzpr3d4t0r @novialriptide @ScriptLineStudios @avaxar

• geometry.rst https://github.com/pygame-community/pygame-geometry/blame/main/docs/line.rst Credits to @Emc2356 @itzpr3d4t0r @ScriptLineStudios @avaxar

• line.c https://github.com/pygame-community/pygame-geometry/blame/main/src_c/line.c Credits to @Emc2356 @itzpr3d4t0r @novialriptide @ScriptLineStudios @avaxar

• geometry.h https://github.com/pygame-community/pygame-geometry/blame/main/src_c/include/geometry.h Credits to @Emc2356 @itzpr3d4t0r @maqa41

• geometry_test.py https://github.com/pygame-community/pygame-geometry/blame/main/test/test_line.py Credits to @Emc2356 @itzpr3d4t0r @novialriptide @ScriptLineStudios @avaxar