Problem:  When setting an option, mapping etc. from Lua without -V1, the
          script ID is set to SID_LUA even if there already is a script
          ID assigned by :source.
Solution: Don't set script ID to SID_LUA if it is already a Lua script.
          Also add _editor.lua to ignorelist to make script context more
          useful when using vim.cmd().

Problem:  When calling an API from Vimscript to set an option, mapping,
          etc., :verbose shows that it's set from an API client.
Solution: Don't override current_sctx.sc_sid when calling an API from
          Vimscript. Also fix the inverse case where API channel id is
          not set when calling an API from RPC. Move channel id into
          sctx_T to make saving and restoring easier.

Related #8329

Add a new field `virt_lines_overflow` that enables horizontal scrolling
for virtual lines when set to "scroll".

Problem: `try_leave()` assertions moved in #31600 no longer hold.
Solution: Remove the assertions.

Problem:  We want to deprecate `nvim_err_write(ln)()` but there is no
          obvious replacement (from Lua). Meanwhile we already have
          `nvim_echo()` with an `opts` argument.
Solution: Add `err` argument to `nvim_echo()` that directly maps to
          `:echoerr`.

Result of `make iwyu` (after some "fixups").

Problem:  Two separate try/end wrappers, that only marginally differ by
          restoring a few variables. Wrappers that don't restore
          previous state are dangerous to use in "api-fast" functions.
Solution: Remove wrappers that don't restore the previous state.
          Always use TRY_WRAP.

Problem:
- API functions using `try_start` directly instead of `TRY_WRAP`, do not
  surface the underlying error message, and instead show generic things
  like "Failed to set buffer".
- Error handling code is duplicated in the API impl, instead of
  delegating to the vim buffer/window handling logic.

Solution:
- Use `TRY_WRAP`.