isLinear check and codepaths for that

[Pomax]

Closes https://github.com/Pomax/bezierjs/issues/148 once finished.

• [x] add linearity check
• [x] add translation-based offsetting for linear segments.
  • [x] with support for graduation
• [x] add outline tests for linear curves
• [ ] ~use the true linear endpoints rather than start/end~ filed as separate issue
• [x] make sure linear segments inside a larger offset curve still work

This also changes the drawShape() function in the graphics API, as end caps were linear cubic curves, which was a bit weird. They're not linear quadratics, which is still a little weird, but at least marginally better.

[Pomax]

Correct cases (taking into account this does not resolve the true start/end coordinates yet):