removing pending virtcol tests

Allow passing in empty array as a shorthand for array with empty string; add more documentation

add check for start_row as well

fix double free because intermediary lines weren't xmemdup'd.

NL-for-NUL dance.

Normalize row indices and perform more validation.

Adjust the cursor position if it's on the right side of the replacement.

Tests and documentation.

Eliminate `expect_err` in favor of `pcall_err` + `eq` or `matches`.

- Rename `meth_pcall`.
- Make `pcall_err` raise an error if the function does not fail.
- Add `vim.pesc()` to treat a string as literal where a Lua pattern is
  expected.

Restore code removed in #9674.

bisected to f5d5da391715

Other test steps:

    nvim -u NORC
    :terminal tree /    " Produces lots of output
    :edit somefile.txt
    :vsplit
    :vsplit
    <c-w>l
    <c-w>l
    <c-w>h
    <c-w>p