Improve support for multiline short codes

[techxplorer]

Thanks for working on #3.

As I noted in a late update to the discussion (errata-ai/vale#789) there is an edge case where the updated regular expression doesn't work. Specifically where there is no trailing whitespace.

My editing environment is set to remove trailing whitespace from lines.

If the multiline short code starts like this, the change in #3 works as expected:

{{% unsplash

If the multiline short code starts like this, it fails:

{{% unsplash

Note the trailing whitespace in the first example, and that it is removed in the second example.

I'm not familiar enough with regular expressions to tell what needs to be changed.

Is there any way it could be updated to support multiline short codes when trailing whitespace is removed?