Warn about using `lexer` as a local in `jade.render()`

Using it as a local in `jade.compile()({lexer: 'blah'})` is still supported.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Merge pull request #1738 from jadejs/real-lexer-warn

Warn about using `lexer` as a local in `jade.render()`