add :comment command for toggling comments

tests/features/comment_test.py

@@ -0,0 +1,75 @@
+from testing.runner import and_exit
+from testing.runner import trigger_command_mode
+
+
+def test_comment_some_code(run, ten_lines):
+    with run(str(ten_lines)) as h, and_exit(h):
+        h.press('S-Down')
+
+        trigger_command_mode(h)
+        h.press_and_enter(':comment')
+
+        h.await_text('# line_0\n# line_1\nline_2\n')
+
+
+def test_comment_some_code_with_alternate_comment_character(run, ten_lines):
+    with run(str(ten_lines)) as h, and_exit(h):
+        h.press('S-Down')
+
+        trigger_command_mode(h)
+        h.press_and_enter(':comment //')
+
+        h.await_text('// line_0\n// line_1\nline_2\n')
+
+
+def test_comment_partially_commented(run, ten_lines):
+    with run(str(ten_lines)) as h, and_exit(h):
+        h.press('#')
+        h.press('S-Down')
+        h.await_text('#line_0\nline_1\nline_2')
+
+        trigger_command_mode(h)
+        h.press_and_enter(':comment')
+
+        h.await_text('line_0\nline_1\nline_2\n')
+
+
+def test_comment_partially_uncommented(run, ten_lines):
+    with run(str(ten_lines)) as h, and_exit(h):
+        h.press('Down')
+        h.press('#')
+        h.press('Up')
+        h.press('S-Down')
+        h.await_text('line_0\n#line_1\nline_2')
+
+        trigger_command_mode(h)
+        h.press_and_enter(':comment')
+
+        h.await_text('# line_0\n# #line_1\nline_2\n')
+
+
+def test_comment_single_line(run, ten_lines):
+    with run(str(ten_lines)) as h, and_exit(h):
+        trigger_command_mode(h)
+        h.press_and_enter(':comment')
+
+        h.await_text('# line_0\nline_1\n')
+
+
+def test_uncomment_single_line(run, ten_lines):
+    with run(str(ten_lines)) as h, and_exit(h):
+        h.press('#')
+        h.await_text('#line_0\nline_1\n')
+
+        trigger_command_mode(h)
+        h.press_and_enter(':comment')
+
+        h.await_text('line_0\nline_1\n')
+
+
+def test_comment_with_trailing_whitespace(run, ten_lines):
+    with run(str(ten_lines)) as h, and_exit(h):
+        trigger_command_mode(h)
+        h.press_and_enter(':comment //   ')
+
+        h.await_text('// line_0\nline_1\n')