testhelpers: Show translated code if NEOVIM_SHOW_TRANSLATED_LUA is set
translator: Assign a: scope values
tests: Add tests for some a: access

Only positive tests currently, no tests for exceptions (as they are not
implemented anyway).
tests: Add :unlet! test
parser: Improve documentation in parser/ex_commands.h
*: Get rid of char_u where possible

Macros near the top of the file that redefine “external” char_u users are some
sort of TODO list that lists things that need to be replaced or adjusted.
*: Replace some strings.h functions with memory.h/string.h ones
*: Save offsets in expression nodes

Saves expression in functions from expression.c in place of parsing newly
allocated string in ex_commands.c.h. One of the alternatives is saving (start,
len) pair in place of (start, end). (start, end) in the current state was a bit
easier to refactor to.
translator/lua: Add support for :let+= operation with lists
translator/lua: Use vim.iter for :for loop

Adds proper error reporting and makes it easier to add iterators or lazy lists.
tests: Add two :for loop tests
parser: Record skips (removed escape characters) in get_cmd_arg

Untested. Will be used for computing error offsets.
testhelpers/lua: Move some helpers code to vim.lua
lua: Check for new/modified globals in test.finish