with `hard_tabs: true` and `tab_size: 4`, shift-alt-down off a
tab-aligned column drops the cursor in the wrong spot:

```
      current.rgb.r         ˇ= p[0] >> 8;      <- cursor here
      residuals[run].rˇgb.r  = p[0] & 0xff;    <- lands here, not on the =
```

both `=` render at column 48 but sit at buffer columns 22 and 27, since
a different number of tabs pads each row. shift-alt-down is
`skip_soft_wrap: true`, which placed the cursor by utf16 column where a
tab counts as one.

so count the column with tabs expanded instead. still a column and not a
pixel position, so the buffer-column behaviour
`test_add_selection_skip_soft_wrap_option` pins doesn't change (without
tabs the expanded column *is* the buffer column). tried pixels first and
it breaks that test, since x is measured inside a wrapped segment.

heads up that the existing tests here all pass `Default::default()`,
which is `skip_soft_wrap: false` (the `default_true` only applies to
deserializing), so nothing covered the branch the keybinding actually
uses.

Closes #60752.

Release Notes:

- Fixed added cursors landing in the wrong column when using tabs to
align code