give module id an invalid name to prevent invalid replacement

[Rich-Harris]

fixes #3526. #3528 was close, but still had the bug — in order to fix it we need the module ID to be something that Rollup can't (in its desire to generate readable code) turn into an identical variable name. We can't use something like {APP} because that seems to cause issues with string replacement (possibly word boundary related? couldn't quite be bothered to figure it out), but if we prefix the ID with a number than Rollup is forced to add another character, thus hacking around the issue.

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[benmccann]

i might add a bit more detail

Suggested change

	'0APP': './server/app.js' // digit prefix prevents CJS build from using this as a variable name
	'0APP': './server/app.js' // digit prefix prevents CJS build from using this as a variable name to ensure replace doesn't replace occurrences of variables with this string