Implement word wrapping in the preview window

Example:
  fzf --preview 'bat --style=plain --color=always {}' \
      --preview-window wrap-word \
      --bind space:toggle-preview-wrap-word

Close https://github.com/junegunn/fzf/discussions/3383

src/tui/tui_test.go

@@ -2,6 +2,46 @@ package tui
 
 import "testing"
 
+func TestWrapLine(t *testing.T) {
+	// Basic wrapping
+	lines := WrapLine("hello world", 0, 7, 8, 2)
+	if len(lines) != 2 || lines[0].Text != "hello w" || lines[1].Text != "orld" {
+		t.Errorf("Basic wrap: %v", lines)
+	}
+
+	// Exact fit — no wrapping needed
+	lines = WrapLine("hello", 0, 5, 8, 2)
+	if len(lines) != 1 || lines[0].Text != "hello" || lines[0].DisplayWidth != 5 {
+		t.Errorf("Exact fit: %v", lines)
+	}
+
+	// With prefix length
+	lines = WrapLine("hello", 3, 5, 8, 2)
+	if len(lines) != 2 || lines[0].Text != "he" || lines[1].Text != "llo" {
+		t.Errorf("Prefix length: %v", lines)
+	}
+
+	// Empty string
+	lines = WrapLine("", 0, 10, 8, 2)
+	if len(lines) != 1 || lines[0].Text != "" || lines[0].DisplayWidth != 0 {
+		t.Errorf("Empty string: %v", lines)
+	}
+
+	// Continuation lines account for wrapSignWidth
+	lines = WrapLine("abcdefghij", 0, 5, 8, 2)
+	// First line: "abcde" (5 chars fit in width 5)
+	// Continuation max: 5-2=3, so "fgh" then "ij"
+	if len(lines) != 3 || lines[0].Text != "abcde" || lines[1].Text != "fgh" || lines[2].Text != "ij" {
+		t.Errorf("Continuation: %v", lines)
+	}
+
+	// Tab expansion
+	lines = WrapLine("\there", 0, 10, 4, 2)
+	if len(lines) != 1 || lines[0].DisplayWidth != 8 {
+		t.Errorf("Tab: %v", lines)
+	}
+}
+
 func TestHexToColor(t *testing.T) {
 	assert := func(expr string, r, g, b int) {
 		color := HexToColor(expr)