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/3752.md

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+```
+% pandoc command/chap1/text.md command/chap2/text.md -f markdown+rebase_relative_paths --verbose -t docx | pandoc -f docx -t plain
+^D
+[INFO] Loaded command/chap1/spider.png from ./command/chap1/spider.png
+[INFO] Loaded command/chap1/../../lalune.jpg from ./command/chap1/../../lalune.jpg
+[INFO] Loaded command/chap2/spider.png from ./command/chap2/spider.png
+Chapter one
+
+A spider: [spider]
+
+The moon: [moon]
+
+Link to spider picture.
+
+URL left alone: manual.
+
+Absolute path left alone: absolute.
+
+Chapter two
+
+A spider: [spider]
+```
+
+```
+% pandoc command/chap1/text.md command/chap2/text.md -f markdown+rebase_relative_paths -t html
+^D
+<h1 id="chapter-one">Chapter one</h1>
+<p>A spider: <img src="command/chap1/spider.png" alt="spider" /></p>
+<p>The moon: <img src="command/chap1/../../lalune.jpg" alt="moon" /></p>
+<p>Link to <a href="command/chap1/spider.png">spider picture</a>.</p>
+<p>URL left alone: <a href="https://pandoc.org/MANUAL.html">manual</a>.</p>
+<p>Absolute path left alone: <a href="/foo/bar/baz.png">absolute</a>.</p>
+<h1 id="chapter-two">Chapter two</h1>
+<p>A spider: <img src="command/chap2/spider.png" alt="spider" /></p>
+```

test/command/chap1/text.md

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+# 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).

test/command/chap2/text.md

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+# Chapter two
+
+A spider: ![spider](spider.png)