chore: tidy up parser

[Rich-Harris]

Started working on #12925 and found a bunch of things we can improve first:

• we should do as much validation during analysis, rather than parsing, as possible — it makes it much easier to (for example) have squigglies that cover a meaningful range, rather than being between two characters
• we're generating every element twice! once for the opening tag, once for the closing tag. the second one just gets discarded. this is silly and wasteful and almost certainly my bad
• the order of stuff in this file is all over the place. tidied it up — consts, export default, helper functions — rather than leaving everything weirdly interleaved

Aside from the squigglies, this doesn't change anything so I'll self-merge once green to unblock

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[dummdidumm]

Another advantage of doing all validation during analysis is that language tools gets a proper Svelte AST in more cases

[Rich-Harris]

great point! will try and move others over if I can, on a drive-by basis