Provide a JavaStringDoubleClickStrategy that treats valid Java
character escapes as "words" to be selected on double click. Install
it for the string, character, and text block document partitions in
the JavaSourceViewerConfiguration.

The strategy depends on the source version because \s is a valid escape
only as of Java 15.

Change-Id: I2cfe61ceb18d5f6bb9e86d7024954afdbc47497b
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.ui/+/184277
Tested-by: JDT Bot <jdt-bot@eclipse.org>
Tested-by: Jeff Johnston <jjohnstn@redhat.com>
Reviewed-by: Jeff Johnston <jjohnstn@redhat.com>