Enable trimDirectiveWhitespaces JSP directive in GSPs

I'm following up on grails/grails-core#4128 to enable trimDirectiveWhitespaces in GSPs. The issue was closed and redirected to this repository, but an issue was never created in grails-gsp.

The [trimDirectiveWhitespaces directive](https://docs.oracle.com/cd/E19316-01/819-3669/bnajm/index.html) is available at page-level for JSP pages:
```<%@page trimDirectiveWhitespaces="true"%>```

We have a rather large HTML page that's rendering 15MB of HTML, though it crunches down to 670KB with gzip. It seems there's a large volume of whitespace introduced by whitespace in GSP templates, so this setting would definitely be useful.

Start by locating the GSP directive-processing entry point and existing tests for page-level directives; no implementation file or test is named in the issue. Compare the requested trimDirectiveWhitespaces behavior with the linked JSP directive documentation, and consider the work done when GSP output trims directive-introduced whitespace and the behavior is covered by tests.

Written by the indexing model from the issue text.