RFR: 8277026: Remove blank lines remaining from snippet markup

[Pavel Rappo]

This PR proposes to omit lines from the snippet source if those lines consist only of markup and perhaps whitespace. For example, if this PR is integrated, these three snippets will result in the same output:

    // @start region="example1"
    if (v.isPresent()) {
        System.out.println("v: " + v.get());
    } // @end


    if (v.isPresent()) { // @start region="example2"
        System.out.println("v: " + v.get());
    } // @end


    // @start region="example3" :
    if (v.isPresent()) {
        System.out.println("v: " + v.get());
    // @end :
    }

While this change complicates the mental model for markup, it removes otherwise unavoidable inner blank lines in properties files, for example:

    coffee=espresso
    # @highlight:
    tea=ice

-------------

Commit messages:
 - Better test and clean up
 - Initial commit for 8277026

Changes: https://git.openjdk.java.net/jdk/pull/6623/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6623&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277026
  Stats: 251 lines in 7 files changed: 240 ins; 2 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6623.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6623/head:pull/6623

PR: https://git.openjdk.java.net/jdk/pull/6623