Skew function

[erap129]

Added skew function (according to issue #536). Very similar to rotation.
Not sure where the documentation is supposed to go (I probably put it in the wrong place)
Opened a new PR instead of the old one that dealt with text only.

Checklist:

• The GitHub pipeline is OK (green),
  meaning that both pylint (static code analyzer) and black (code formatter) are happy with the changes of this PR.

• A unit test is covering the code added / modified by this PR

• This PR is ready to be merged

• In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder

• A mention of the change is present in CHANGELOG.md

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

[Lucas-C]

This is really great!
I'm going to perform a new release once this nice addition has landed on the master branch 😃
Thank you for your contribution @erap129

[Lucas-C]

@allcontributors please add @erap129 for code

[allcontributors]

@Lucas-C

I've put up a pull request to add @erap129! 🎉

[Lucas-C]

Thank you @erap129 for your contribution!
Good job

[Lucas-C]

Out of curiosity, what made you want to contribute this PR to fpdf2? 😊

[erap129]

Out of curiosity, what made you want to contribute this PR to fpdf2? blush

I reached you through the up-for-grabs hashtag and it seemed like a cool repo :)

[Lucas-C]

I reached you through the up-for-grabs hashtag and it seemed like a cool repo :)

Ahaha, great! 😄