fix: added support legacy browsers in preload code

[NikolayMakhonin]

Description

Replaced modern js features in preload code with their legacy equivalents for support legacy browsers.

Additional context

The initial purpose is to add support legacy browsers to the @svelte/kit. You can see my template project for this:
https://github.com/NikolayMakhonin/svelte-kit-template

I have tested these changes for svelte/kit in Chromium 39 and 44. It worked. But I'm not sure if the same problem won't happen again in the future for other frameworks.

The most frequent modern js features, that I had replace in defferent projects for support legcy browsers:

1. use anonymous functions instead arrow functions
2. use var instead let / const
3. use regexp instead startsWith / endsWith
4. don't use Array.from
5. don't use string templates
6. don't use spread operators or shorthand properties

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

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[NikolayMakhonin]

Solved in this issue. This pull request will be worse than the solution described there.