Problem:    getcwd() is unclear about how 'autochdir' is used.
Solution:   Update the help for getcwd().  Without any arguments always return
            the actual current directory. (closes vim/vim#9142)
https://github.com/vim/vim/commit/851c7a699ae00bdc14a4db874cf722b7b7393b53

For anonymous scripts, defer the creation of script items until an attempt to access a script-local
variable is made. This dramatically reduces the number of script items created when using lots of
vim.cmd and nvim_exec especially.

This will mean <SID> usage fails until a script-local variable access is first made.

Based on #13143 (and #11507) with changes:

 - Omit script_type_E. Use sn_name == NULL to determine anon items.
 - Keep SID_STR. Used by anon :source for .lua files (no item).
 - Show SID in get_scriptname output (:verbose set).
 - Factor item creation into new_script_item.
 - Leave sc_seq = 0 (anon scripts don't re-use the same item when re-sourced).
 - Add tests for anon :source.

Co-authored-by: Vikram Pal <vikrampal659@gmail.com>
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

Problem
- `redir_exec` is obsolete, but it keeps getting used in new tests
  because people copy existing tests.
- Disadvantages of `redir_exec`:
  - Captures extra junk before the actual error/message that we _want_ to test.
  - Does not fail on error, unlike e.g. `command()`.

Solution
- Use new functions like `nvim_exec` and `pcall_err`.

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