Add testtextface devtest mod with comprehensive test nodes

Test nodes covering supported configurations:

- Normal node (top face, side face)
- Nodebox slab
- Signlike (wall-mounted, full-face)
- Mesh cube, mesh slanted display
- Wide sign (3 nodes wide, 1024x341 resolution)
- Mono font, bold font
- Color test (rainbow text)
- Entity with text (cube entity, spawner item)

All nodes have right-click text editing with formspec controls for
font size, face, weight, alignment, text color, and background color.

Includes testtextface.make_text_texture() Lua helper for constructing
[text: modifier strings for use on entities.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sofar