More concise exemption syntax

[fabio-looker]

Rather than having to write an exemption like:

#LAMS
#rule_exemptions: {K1: "reason"}

We can remove the line-break requirement for conditional comments, and expose a more concise syntax:

#LAMS exempt: K1 {why: "reason"}

The syntax no longer forces you to provide a reason, so it could be as succinct as:

#LAMS exempt: K1 {}

However, it is easy to instead simply require reasons via a rule, if desired.

PS. While tweaking how conditional comments are handled, also handle: https://github.com/looker-open-source/look-at-me-sideways/issues/137

[fabio-looker]

Implemented in v3.4