Protect language converter markup in the preprocessor (take 2).

Protect language converter markup in the preprocessor (take 2).

This revises 28774022769d2273be16c6c6e1cca710a1fd97ef, which was
reverted in master due to unexpected issues with `-{{...}} ` markup
on translatewiki and enwiki.  Test cases are added to ensure that this
is parsed as a template, not as language converter markup.

https://www.mediawiki.org/wiki/Preprocessor_ABNF is the canonical
documentation for the preprocessor; this will be updated after this
patch is merged.  The basic principles described in that page are
maintained in this patch:

* Rightmost opening structure has precedence: `-{{` is parsed as a
dash followed by template opening.

* `{{{` has precedence over `{{` and `-{`: `-{{{{` is parsed as
`-{` `{{{` since we first grab the rightmost `{{{`.

A bunch of test cases were added to verify the "ideal precedence"
order described on that wiki page.

This patch introduced some minor incompatibilities in existing
markup, in particular with chemical formulae in templates.
Fixes for these are being tracked at
https://www.mediawiki.org/wiki/Parsoid/Language_conversion/Preprocessor_fixups

Bug: T146304
Bug: T153761
Change-Id: I2f0c186c75e392c95e1a3d89266cae2586349150