Option to insert comment character(s) at the beginning of the line(s) (#19746)

Closes #19459


This PR adds the optional setting to insert comment character(s) at the
beginning of the line(s) instead of after the indentation. It can be
enabled via keybindings:

```
"ctrl-/": ["editor::ToggleComments", { "ignore_indent": true }]
```

As suggested by @notpeter in #19459, this is implemented in
`toggle_comments` (editor.rs) taking the existing `advance_downwards`
option as example.

There's also a test case for the setting, which mimics the test case for
the regular comment toggling behavior.

---

I am not entirely happy with the name `ignore_indent`. The default would
be a double negative now `ignore_indent=false`. A positive wording would
probably easier to understand, but I could not think of anything
concise. `insert_at_line_start` or just `at_line_start` might work, but
didn't convince me either. That said, I am happy to change the name if
there are better ideas.

---

Release Notes:

- Added optional setting to insert comment character(s) at the beginning
of the line(s) instead of after the indentation. It can be used by
changing the default mapping to toggle comments like this: `"ctrl-/":
["editor::ToggleComments", { "ignore_indent": true }]`