Textcanvas

- Add 'Plot::bars()' and 'Chart::bars() to render bar plots and charts.
- Add 'Plot::function_filled()' and 'Chart::function_filled()' to render
functions with the area under the curve filled-in.
- Add 'Quick Start' section to README.
- Python: Refactor tests to use multi-line strings for renders (looks
cleaner and matches the Rust version).
- Rust: Mark env-altering test blocks 'unsafe' to prepare for Rust 2024.
- Minor code refactorings.

- Add vector math helpers, to do simple operations on 2D vectors (e.g.,
+, -, /, *, sum, mean, magnitude, normalize, normal, dot products, or
projections).
- Add interpolation helpers, to compute simple interpolations (e.g.,
linear, ease in/out/in-out, smoothstep, or Catmull-Rom).
- Support no-color mode. This mode is entered by setting the 'NO_COLOR'
environment variable to a non-empty value. This is implemented at the
level of 'Color'; anything using 'Color' will benefit from this.