…on headings

When a CHANGELOG.md file starts directly with a version heading (e.g. ## 1.3.0)
rather than a package title heading (e.g. # my-package), new version entries were
incorrectly inserted after the existing version heading line instead of before it.
This caused changelogs like:

  ## 1.3.0         <- existing
  ## 1.4.0         <- new entry, wrongly placed under old heading
  ### Minor Changes
  ...
  ### Minor Changes <- old content appears to be under new heading
  ...

The fix detects whether the first line of the changelog is a version heading using
/^#{1,6}\s+\d+\.\d+/ regex. When it is, the new entry is prepended before the
entire file content instead of after the first line.

Fixes changesets#1056