Add test framework with VT100 terminal emulator

Implement comprehensive test suite for linenoise:

VT100 Terminal Emulator:
- Column-based UTF-8 representation with display width tracking
- Each screen cell stores complete UTF-8 characters (up to 32 bytes)
- Proper handling of wide characters (emoji, CJK) as 2-column cells
- ZWJ tracking for grapheme cluster storage
- Escape sequence parsing for cursor movement and screen clearing

Test Harness:
- Fork/pipe architecture for testing via LINENOISE_ASSUME_TTY
- Visual rendering to real terminal for debugging failed tests
- Assertion helpers for screen content and cursor position

Test Coverage (72 tests):
- Basic typing and cursor movement
- UTF-8 input and navigation (é, 中, 🎉)
- Emoji cursor movement and deletion
- Grapheme clusters (heart+VS, thumbs up+skin tone, rainbow flag)
- Horizontal scrolling with long lines
- Horizontal scrolling with UTF-8 content
- Multiline mode basics
- Multiline history navigation (regression test)
- Word/line deletion (Ctrl-W, Ctrl-U)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Makefile

@@ -3,5 +3,11 @@ linenoise_example: linenoise.h linenoise.c
 linenoise_example: linenoise.c example.c
 	$(CC) -Wall -W -Os -g -o linenoise_example linenoise.c example.c
 
+linenoise-test: linenoise-test.c linenoise_example
+	$(CC) -Wall -W -Os -g -o linenoise-test linenoise-test.c
+
+test: linenoise-test linenoise_example
+	./linenoise-test
+
 clean:
-	rm -f linenoise_example
+	rm -f linenoise_example linenoise-test