Disable the embedded syntax feature of newer syntax/rst.vim:

- It doesn't work with both "c" and "cpp" active simultaneously, since both
  rely on including syntax/c.vim, and the double inclusion of this file
  causes problems.

- It requires a fairly new Vim, and we'd like to support older ones, too.

- It marks the block with NoSpell, which we don't want.

- It's easier to disable the support in syntax/rst.vim entirely than to
  partially use it and work around its limitations.

Also:

- Define variable g:rstEmbeddedLangs to allow users to control which
  languages are embedded.