The parser was too strict here and would reject valid constructs
that used an unescaped `$` that was not part of a variable
expression (and not ambiguous).

Now, only errors for unmatched braced expressions (e.g. `${FOO`)
are returned, but other valid cases are ignored and the `$` will
be treated literally, e.g. `a $ string` -> `a $ string`, which is
the same as in POSIX.

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

Signed-off-by: Guillaume Lours <705411+glours@users.noreply.github.com>

* Handle standard escape sequences based on XSI/XBD
  https://pubs.opengroup.org/onlinepubs/9699919799/utilities/echo.html
* Fix parsing with escaped quotes at end of quoted
  value
* Handle key whose name starts with `export` (require
  whitespace between `export` keyword when stripping)
* Fix parsing of unquoted values with embedded `#`
  (require a space before starting an inline comment)
* Use correct parser methods throughout all test cases
* Eliminate a lot of unused code that was not working
  right

Signed-off-by: Milas Bowman <milas.bowman@docker.com>