Use custom parser instead of postcss-safe-parser

[TrevorBurnham]

This PR effectively reverts 20e5242, which broke an existing test case. 😓 It also changes the parser used by the postcss plugins, which fixes a new test case and I think the case that 20e5242 was intended to solve (must confirm!).

Explanation

When postcss-safe-parser sees code like

&& {
  color: black;
  PLACEHOLDER_PATTERN;
}

it sees that PLACEHOLDER_PATTERN; isn't valid CSS syntax, so it treats it as whitespace. This is a problem for us because postcss-nested makes no effort to keep whitespace in the same block (or any block, for that matter), yielding

.namespace && {
  color: black;
}

PLACEHOLDER_PATTERN;

The fix here is to use a modified version of postcss-safe-parser that treats these "unknown words" as declarations instead, yielding

.namespace && {
  color: black;
  PLACEHOLDER_PATTERN;
}