Use esbuild as a JavaScript minifier

What does this MR do?

esbuild provides a much higher performance than the default Terser plugin. We could potentially use esbuild instead of babel as well, but there are currently two caveats where babel wins:

• We need to pre-process our JS for webpack@4, as it uses an older version of acorn.
• We automatically polyfill based on babel-preset-env

Preliminary tests have shown that even the minification step could reduce compilation times more than half, on my machine 12 => 4 minutes.

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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