Support .xyz/.rgb + swizzling in ti.Vector

[k-ye]

I think we can specialize up to 4-dimensional vectors by supporting these features:

• .uv, xyz, rgba: These are fields with physical meanings.
• vector swizzle: https://en.wikipedia.org/wiki/Swizzling_(computer_graphics). Another feature commonly used in GLSL

cc @bobcao3 @neozhaoliang @strongoier

[neozhaoliang]

Fully agree. Emancipating the productive forces for graphics. 😆

Another important feature in GLSL is the initialization by concatenating low dimension vectors, e.g. vec4(vec2, vec2). Could be tricky to do so in python.

• vec2: uv, xy
• vec3: xyz, rgb
• vec4: xyzw, rgba

[k-ye]

Done in #4810