only run postinstall if package.json exists

[Rich-Harris]

• closes Postinstall breaks when package.json is missing (for instance on a pnpm fetch) #6987
• closes postinstall fails if workspaces uses object notation #7000

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[Conduitry]

Should we have another bigger try-catch here in case there are other weird unexpected errors? Given what we're using the postinstall script for here, it doesn't seem to me there would ever be a situation where I would prefer the whole install to fail.

[Rich-Harris]

In general I'm not a fan of all-encompassing try-catch blocks, because they prevent us from catching actual logic bugs. Like, the thing in #6987 (comment) with workspaces not being an array of strings — if we just wrapped the whole thing then we wouldn't encounter that situation, we'd just bail out. I guess we could print the error and still exit with code 0

[Conduitry]

Logging the exception sounds fine. I'm just saying that we shouldn't fail the whole install, because that doesn't help anyone, especially for something that would be generated when they build the project anyway.