Performance improvement: avoid useless call to esbuild to re-transpile TS files

[Lucas-C]

First, thank you for developping & maintaining this great tool 👍

Description

I'm using artillery in a TypeScript projet, and I noticed repeated log lines like this:

Bundled Typescript file into JS. New processor path: .../functions.js
Bundled Typescript file into JS. New processor path: .../functions.js
Bundled Typescript file into JS. New processor path: .../functions.js

I checked the source code in prepare-test-execution-plan.js and realized that repeated transpilation of the same TS file where occuring.

First, a function named transpileTypeScript() has been introduced at some point, and it could be used in replaceProcessorIfTypescript() to avoid repeating code, so this light refactoring is included in this PR.

Second and more importantly, given that transpileTypeScript() is already called beforehand in prepareTestExecutionPlan() at the beggining of a test execution, invoking it again in replaceProcessorIfTypescript() does not seem necessery.
In this MR I added a test so that the transpiling step is skipped if the JS output file already exists in the dist/ directory.
But maybe this transpiling step is simply never needed, you probably know better than me 🙂

Anyway, this PR provides a very substantial performance improvement when using artillery with TS hooks.

Pre-merge checklist

This is for use by the Artillery team. Please leave this in if you're contributing to Artillery.

• Does this require an update to the docs?
• Does this require a changelog entry?

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[Lucas-C]

Note: I simply move/preserved this pre-existing comment there.

[Lucas-C]

Note: this return value was never used so I simply removed it.