Ensure that syntax highlighting works (for a select few languages)

[axic]

Currently some EIPs use "random" capitalization of the language for syntax highlighting. I thought markdown renderers would lowercase those and essentially allow any capitalization. It turns out this is not the case, at least not with Kramdown/Jekyll:

• https://eips.ethereum.org/EIPS/eip-1337 has "```SOLIDITY" and it is not highlighted
• https://eips.ethereum.org/EIPS/eip-2535 has "```Solidity" and it is not highlighted

Some others use JavaScript and js, though I think the js shorthand works.

I'd would suggest to add a check for syntax highlighting to only use lowercase letters.