add CollapseWhitespace for flattening runs of whitespace

CollapseWhitespace turns every run of whitespace into a single space and
trims the ends, so a block of text becomes one tidy line. Comes with a doc
comment and table-driven tests. (closes #7)

Model: opus-5

whitespace_test.go

@@ -0,0 +1,36 @@
+package util
+
+import "testing"
+
+// Written as a code point so that it is visible in the source rather than
+// looking like an ordinary space.
+const nonBreakingSpace = string(rune(0x00a0))
+
+func TestCollapseWhitespace(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{"nothing to change", "one two", "one two"},
+		{"leading and trailing spaces", "  one two  ", "one two"},
+		{"run in the middle", "one    two", "one two"},
+		{"tabs and newlines", "one\t\ttwo\nthree", "one two three"},
+		{"windows line endings", "one\r\ntwo", "one two"},
+		{"non-breaking space", "one" + nonBreakingSpace + "two", "one two"},
+		{"mixed whitespace run", "one \t" + nonBreakingSpace + " two", "one two"},
+		{"only whitespace", " \t\n ", ""},
+		{"empty string", "", ""},
+		{"single word with padding", "\tword\n", "word"},
+		{"paragraph becomes one line", "first line\n\nsecond line\n", "first line second line"},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			got := CollapseWhitespace(test.input)
+			if got != test.expected {
+				t.Errorf("expected %q got %q", test.expected, got)
+			}
+		})
+	}
+}