Add rebase_relative_paths extension.

- Add manual entry for (non-default) extension
  `rebase_relative_paths`.
- Add constructor `Ext_rebase_relative_paths` to `Extensions`
  in Text.Pandoc.Extensions [API change]. When enabled, this
  extension rewrites relative image and link paths by prepending
  the (relative) directory of the containing file.
- Make Markdown reader sensitive to the new extension.
- Add tests for #3752.

Closes #3752.

NB. currently the extension applies to markdown and associated
readers but not commonmark/gfm.

test/command/chap1/text.md

@@ -0,0 +1,11 @@
+# Chapter one
+
+A spider: ![spider](spider.png)
+
+The moon: ![moon](../../lalune.jpg)
+
+Link to [spider picture](spider.png).
+
+URL left alone: [manual](https://pandoc.org/MANUAL.html).
+
+Absolute path left alone: [absolute](/foo/bar/baz.png).