fix: always lowercase HTML elements, for XHTML compliance

[Rich-Harris]

Another XHTML thing, per #17418 (comment) (that PR doesn't address this issue)

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[7nik]

I fact I mentioned this one in the discord:

In XHTML tag names must be lowercased.
Also, XHTML attributes must be lowercased as well to work - not sure what to do here.

I doubt we really need it and I'm agree with @paulocoghi about leaving the lowercasing in XHTML to the user's responsibility

you know if you are writing XHTML and if you purposefully write code that doesn't adhere with what you are serving and expect svelte to correct that screw you 😄

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In the #17418 (comment) I meant the runtime check, which that PR address but incorrecly because <iNpUt> won't be rendred and behave as the input in XHTML, only <input>