Code highlighting language specifier is case sensitive

here: https://github.com/nuxt-content/mdc/blob/85ac40abdf2683172eed9adb64723b501059cdbc/src/runtime/highlighter/shiki.ts#L78

lang is never normalized (e.g. .toLowerCase()), and has given me problems when I use `TS` (uppercase) for the codeblock instead of `ts` (lowercase). Perhaps we should normalize lang.