Shared: makeSections behavior changes.

+ When the optional base level parameter is provided, we no
  longer ensure that the sequence of heading levels is gapless
  [behavior change]. Instead, we set the lowest heading level to
  the specified base level, and adjust the others accordingly. If
  an author wants to skip a level, e.g. from level 1 to level 3,
  they can do that. In general, the heading levels specified
  in the source document are preserved; `makeSections` only
  puts them into a hierarchical structure. Closes #9398.

+ Section numbers are now assigned so that the top level
  gets `1`, no matter what heading level is used. So, even
  if the top heading level is 2, numbers will be `1`, `2`, etc.
  rather than `0.1`, `0.2`, as in the past. Closes #5071.

+ We revert to the old behavior when the `--number-offset` option
  is used. So, for example, if a document begins with a level-3
  heading, and `--number-offset=1,2` is used, the top-level section
  numbers will be `1.2.1`, `1.2.2`, etc. This is mainly for
  backwards-compatibility.

pandoc-lua-engine/test/lua/module/pandoc-structure.lua

@@ -48,7 +48,7 @@ return {
       assert.are_equal('Div', hblks[1].t)
       assert.are_equal('Header', hblks[1].content[1].t)
       assert.are_equal('1',   hblks[1].content[1].attributes['number'])
-      assert.are_equal('1.1', hblks[1].content[3].attributes['number'])
+      assert.are_equal('1.0.1', hblks[1].content[3].attributes['number'])
     end)
   },