runtime(vim): Update base-syntax, improve script-interface command highlighting

runtime(vim): Update base-syntax, improve script-interface command highlighting

- Normalise interface heredoc highlighting with that used for
  :let-heredocs.
- Remove interface feature testing.  The Lua and Python interface
  command scripts are now highlighted by default.  Loading all syntax
  files incurs an undesirable load-time burden so highlighting of the
  less popular MzScheme, Perl, Ruby and Tcl interfaces is disabled by
  default.  g:vimsyn_embed can still be used to customise the supported
  interfaces.
- Always highlight interface ex-commands as valid commands, even when
  the corresponding command-script highlighting is disabled.
- Highlight simple command-script statements as well as heredocs.
- Remove error highlighting of heredoc and statement command-script
  regions when an interface is disabled.  These are now highlighted as
  plain text.
- Allow indented heredoc end tokens when "trim" is specified.
- Match interface heredocs in :def functions.
- Fix runaway vimEmbedError regions.  These regions have been removed.
- Use python2 syntax for :python, and :pythonx when 'pyxversion' is
  appropriately set.

closes: #15522

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>