[SE-0354] Update proposal (#1673)

* [SE-0354] Update proposal

- Specify the behavior around `(?-x)` and `\Q...\E`
- Add additional alternatives considered

* [SE-0354] Separate out alternatives that deal with alternative delimiters

The preamble here only applies to some of the
alternatives under consideration here.

* [SE-0354] Update multi-line semantic alternative

Re-work the section, and discuss full support of
matching option changing in the delimiter

* [SE-0354] Add trailing whitespace alternative

* [SE-0354] Update proposal to include trailing space rule

Author: hamishknight