Fix TextLength to count characters instead of bytes

Use utf8.RuneCountInString so multi-byte characters like é count as one
character. Add a unicode test case to cover this.

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/prosemirror/node_test.go

@@ -487,6 +487,17 @@ func TestTextLength(t *testing.T) {
 		},
 	)
 
+	t.Run(
+		"multi-byte unicode characters",
+		func(t *testing.T) {
+			t.Parallel()
+			raw := `{"type":"doc","content":[{"type":"paragraph","content":[{"type":"text","text":"café résumé"}]}]}`
+			doc, err := Parse(raw)
+			require.NoError(t, err)
+			assert.Equal(t, 11, doc.TextLength())
+		},
+	)
+
 	t.Run(
 		"testdata document",
 		func(t *testing.T) {