Add svn repository browser support

[didiez]

Added a high-level DSL option to configure a repository browser for svn SCM.
To this day, a svn browser could only be configured modifying the underlying XML with a configure block.

svn {
    location('https://svn.didiez.es/')
    configure { scmNode ->
        scmNode / browser(class: 'hudson.scm.browsers.WebSVN') {
            url('https://didiez.es/websvn/')
        }
    }
}

Usage example:

New high-level DSL:

svn {
    location('https://svn.didiez.es/')
    browser {
        webSVN('https://didiez.es/websvn/')
    }
}

or with Dynamic DSL:

svn {
    location('https://svn.didiez.es/')
    browser {
        webSVN {
            url('https://didiez.es/websvn/')
        }
    }
}

[basil]

Does the Dynamic DSL not work here? If not, could it be made to work?