Add minimal support for SVG conversion and probing

[benoit74]

Fix #113

Changes:

• add a new convert_svg2png function to convert SVG to PNG
  • this does not reuse the existing convert_image API for three reasons. First it would make the code of convert_image more complex since we would have to first probe for image format to know if it is SVG (this is for now handled internally in PIL library, but does not work for SVG). Second because the conversion is almost always requiring to pass output width/height and these are different parameters for CairoSVG than for PIL, so the "user" already need to know that he is converting a SVG. Third because current need (convert SVG icon of websites in warc2zim) is only focused on conversion to PNG while convert_image is far more generic ; and CairoSVG does not have a generic conversion function, only conversion to a given format.
• add support for probing SVG images
  • this is not handled by PIL, so we fallback to use libmagic since this library is already used in scraperlib and probably the most reliable/standard way to detect an SVG

Nota: https://github.com/openzim/python-scraperlib/pull/178 must be reviewed and merged first, these changes are based on this other PR

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (1eddabc) to head (c8e2a08).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #181 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 32 32 Lines 1452 1501 +49 Branches 251 262 +11 ========================================= + Hits 1452 1501 +49 ```

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