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>

runtime/syntax/testdir/input/vim_ex_python.vim

@@ -0,0 +1,159 @@
+" Vim :python, :pydo and :pyfile commands
+" Vim :python3, :py3do and :py3file commands
+" Vim :pythonx, :pyxdo and :pyxfile commands
+" VIM_TEST_SETUP let g:vimsyn_embed = "P"
+" VIM_TEST_SETUP let g:vimsyn_folding = "fP"
+" VIM_TEST_SETUP setl fdc=2 fdl=99 fdm=syntax
+
+
+lua << EOF
+print("Lua script")
+EOF
+
+  lua << trim EOF
+    print("Lua script")
+  EOF
+
+lua <<
+print("Lua script")
+.
+
+  lua << trim
+    print("Lua script")
+  .
+
+function Foo()
+  lua << trim EOF
+    print("Lua script in :func")
+  EOF
+endfunction | call Foo()
+
+def Bar()
+  lua << trim EOF
+    print("Lua script in :def")
+  EOF
+enddef | call Bar()
+
+lua print("Lua statement")
+      "\ comment
+      \ print("Lua statement again")
+
+luado print("Lua statement")
+      "\ comment
+      \ print("Lua statement again")
+
+luafile foo.lua
+
+
+" :mzscheme and :mzfile
+
+mzscheme << trim EOF
+  (display "MzScheme script")
+EOF
+
+mzscheme (display "MzScheme statement")
+      "\ comment
+      \ (display "MzScheme statement again")
+
+mzfile foo.rkt
+
+
+" :perl and :perldo
+
+perl << trim EOF
+  print("Perl script\n")
+EOF
+
+perl print("Perl statement\n");
+      "\ comment
+      \ print("Perl statement again\n")
+
+perldo print("Perl statement\n");
+      "\ comment
+      \ print("Perl statement again\n")
+
+
+" :python, :pydo and :pyfile
+
+python << trim EOF
+  print("Python script")
+EOF
+
+python print("Python statement");
+      "\ comment
+      \ print("Python statement again")
+
+pydo print("Python statement");
+      "\ comment
+      \ print("Python statement again")
+
+pyfile foo.py
+
+
+" :python3, :py3do and :py3file
+
+python3 << trim EOF
+  print("Python3 script")
+EOF
+
+python3 print("Python3 statement");
+      "\ comment
+      \ print("Python3 statement")
+
+py3do print("Python3 statement");
+      "\ comment
+      \ print("Python3 statement")
+
+py3file foo.py
+
+
+" :pythonx, :pyxdo and :pyxfile
+
+pythonx << trim EOF
+  print("PythonX script")
+EOF
+
+pythonx print("PythonX statement");
+      "\ comment
+      \ print("PythonX statement")
+
+pyxdo print("PythonX statement");
+      "\ comment
+      \ print("PythonX statement")
+
+pyxfile foo.py
+
+
+" :ruby, :rubydo and :rubyfile
+
+ruby << trim EOF
+  puts "Ruby script"
+EOF
+
+ruby puts "Ruby statement";
+      "\ comment
+      \ puts "Ruby statement again"
+
+rubydo puts "Ruby statement";
+      "\ comment
+      \ puts "Ruby statement again"
+
+rubyfile foo.rb
+
+
+" :tcl, :tcldo and :tclfile
+
+tcl << trim EOF
+  puts "TCL script"
+EOF
+
+tcl puts "TCL statement";
+      "\ comment
+      \ puts "TCL statement again"
+
+tcldo puts "TCL statement";
+      "\ comment
+      \ puts "TCL statement again"
+
+tclfile foo.tcl
+