Fix build error caused by mkdocs (v1.5.0)

[mgao6767]

Fix https://github.com/timvink/mkdocs-charts-plugin/issues/16#issue-1823961327 as suggested by mkdocs' release note:

Breaking change: config.extra_javascript is no longer a plain list of strings, but instead a list of ExtraScriptValue items. So you can no longer treat the list values as strings. If you want to keep compatibility with old versions, just always reference the items as str(item) instead. And you can still append plain strings to the list if you wish.

[SH2282000]

I would rather write list(map(str, config["extra_javascript"])) instead of [str(item) for item in config["extra_javascript"]] as the same function (str()) is applied to all elements.

This is a breaking change, would it be possible to review this M.R. A.S.A.P. ?

cc @timvink @mgao6767 @squidfunk

[squidfunk]

I'm not sure why I was CC'd.

[timvink]

Thanks for the efforts @mgao6767 and @oprypin !