Support variable expansion also in placeholders, like in
`{unsetPlaceholder:default value}`. Implemented similarly to caddyserver#3682,
which provided defaults for environment variable expansion.

Closes caddyserver#1793.

Signed-off-by: Jens Erat <jens.erat@mercedes-benz.com>

If we have a string `\}`, we'd expect this to be interpreted as `}`, similar to how `\}\{` would be interpreted as `}{`. If we optimize early by searching for opening braces, we'd not strip the escape character here.

Signed-off-by: Jens Erat <jens.erat@mercedes-benz.com>

The old parser logic was quite complicated and does not support nested placeholder evaluation for defaults, as does not support counting closing braces.

I assume the given approach using two loops and switch statements is more readable, and changes braces matching behavior to support nested defaulting provided in another commit.

Signed-off-by: Jens Erat <jens.erat@mercedes-benz.com>

Signed-off-by: Jens Erat <jens.erat@mercedes-benz.com>