Throw syntax errors when parsing invalid input

[haines]

At the moment, if parse encounters valid JSON in an unexpected format, it may either throw a confusing error, or silently return the wrong output.

parse('"hello"') // => 'h'
parse('42') // =>  ReferenceError: Cannot access 'hydrated' before initialization
parse('true') // => undefined
parse('null') // => TypeError: Cannot read properties of null (reading 'length')
parse('{}') // => undefined
parse('[]') // => undefined

This PR introduces checks for unexpected values before hydrating and throws a SyntaxError if the input cannot be parsed.

[Rich-Harris]

Thank you! Since devalue.parse needs to be as small as possible (as it's designed to be used in the browser to hydrate server payloads), I made a few tweaks. The expectation is that you'd always be using it with the output of devalue.stringify, so if the input is incorrect, the specific way in which it's incorrect doesn't really matter — it just means that things haven't been wired up correctly. As such, I think a generic Invalid input error is sufficient, given that it results in a smaller library than the more descriptive errors.

[haines]

Sounds good, thanks!