PicoTurtle sample programs

The examples live in lua/samples. Run one from the repository root:

./build/picoturtle lua/samples/polygons.lua

Sample	Purpose
`turtle_basic.lua`	Minimal movement
`polygons.lua`	Repeated regular polygons
`spiral.lua`	Incremental movement and turning
`tree.lua`	Recursive branching
`sierpinski.lua`	Recursive geometry
`fern.lua`	Larger recursive drawing and Latin text
`text-demo.lua`	Rotated Noto Sans text
`save_restore.lua`	Turtle state stack
`draw_mandrill.lua`	Drawing an image path
`draw_mandrill_move.lua`	Reusable images and animation