js-tokens: Use explicit source preload

[camillobruni]

Using the function source is technically not defined to always return the source string.
Additionally this forces chrome to use a special external string type since it's rather uncommon to access the function source.

Until we can fix the resource loading to handle duplicate entries for files and preload data we will duplicate the js-tokens.js file to get a separate name.

Addresses issue #271.

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[camillobruni]

I actually mis-mesured since I was doing this on the command line instead of the browser where we actually the full external string implementation:

Before (10 runs):

browser          Google Chrome Beta     Safari Technology Preview  Firefox
version          145.0.7632.26          26.0.0.234.20624.1.6.19.3  147.0.1
cpu              Apple M1 Max 10 cores  Apple M1 Max 10 cores      Apple M1 Max 10 cores
js-tokens/Score  186.0 ± 3.2%           328 ± 4.3%                 281.9 ± 1.2%
Score            186.0 ± 3.2%           328 ± 4.3%                 281.9 ± 1.2%

After (10 runs):

browser          Google Chrome Beta     Safari Technology Preview  Firefox
version          145.0.7632.26          26.0.0.234.20624.1.6.19.3  147.0.1
js-tokens/Score  347.2 ± 1.3%           324 ± 5.1%                 213.3 ± 0.60%
Score            347.2 ± 1.3%           324 ± 5.1%                 213.3 ± 0.60%