It's possible for weirdness to happen if curbuf is modified while
sourcing from it via :source (with no arguments). For example:

- Deleting lines from or wiping curbuf can cause internal error E315 to
  be thrown from ml_get.
- Changing the curbuf to another buffer while sourcing can cause lines
  from the new curbuf to then be sourced instead.

Problem:
Anonymous :source (no args) and nvim_exec() don't support Vimscript line continuations.

Solution:
Factor out the concat logic into concat_continued_line() and a
CONCAT_CONTINUED_LINES macro for simple concatenations where lines are
fetched individually.

Closes #14807

Ensure that
  * Shell uses UTF8 input/output mode
  * Stderr output is captured, in UTF8
  * Program exit codes are correctly captured

Update functional test harness and add tests
for :make command.

Closes #13713

It's happening because do_source is only expected to return FAIL when it
was unable to open file . But `nlua_exec_file` returns fail for parsing
and execution error too . Those errors are emitted through `nlua_error`.

So now return value of nlua_exec_file is ignored like do_cmdline. It now
only returns fail when it was unable to open file that check is done
before calling nlua_exec_file or do_cmdline. Errors in nlua_exec_file
are still directly emitted through nlua_error like before.

So now :source can run lua files too :)

* feat: Add support for :[ranged]source for lua files

Fixes https://github.com/neovim/neovim/issues/11188.

Problem:    All tab stops are the same.
Solution:   Add the variable tabstop feature. (Christian Brabandt,
            closes vim/vim#2711)
https://github.com/vim/vim/commit/04958cbaf25eea27eceedaa987adfb354ad5f7fd

Fix https://github.com/neovim/neovim/issues/8722