Changed lang attribute to data-lang for AAA in screen readers

[CarlosLongarela]

Please check that all is ok (I made the edits in Github).

Finally, my other Slack comment about AAA spec contrast:

• I changed prism.css to https://github.com/PrismJS/prism-themes/blob/master/themes/prism-a11y-dark.css (and changed line-height to 1.2) because these styles are fully a11y compatible (https://www.a11yproject.com/) if you maintain actual prism.css file, maybe would be an excellent option to mentions this CSS in the readme file (light version also available https://raw.githubusercontent.com/ericwbailey/a11y-syntax-highlighting/master/dist/prism/a11y-light.css)

Thanks a lot.

[mkaz]

This is going to be a breaking change to existing blocks by changing how attributes are parsed. I need to consider it carefully and also will need to include deprecation to migrate attributes seamlessly.

Good suggestion with the a11y color schemes, I created issue #84 that can address. I've been meaning to allow for multiple color themes for awhile just haven't gotten to implementing.

[CarlosLongarela]

Ok, understand the compatibility problem, ¿maybe search for data-lang attribute, and if missing, search for lang? Maybe with that approach, all new code blocks would be data-lang and old code blocks as lang, but the user could change it to data-lang without breaking the block.

Issue #84 sounds good

[mkaz]

Hi Carlos, I used your commits and created PR #85 - I extended and added the deprecated property.

Unfortunately, it's not working as expected and I'm still getting an invalid block when opening a block created previously that uses the old attribute. I'm not sure if this is because of how I'm extending a core block or not.

[mkaz]

Closing in favor of #85 (if we can get that one to work)