Update docs re: moduleFileExtensions to add ordering note (left-to-right)

[jeffgore00]

Adds note that the moduleFileExtensions configuration evaluates the array of file extensions from left-to-right. This may seem self-evident, but personally after dealing with Webpack loaders which work in the opposite direction, I think it doesn't hurt to point out.

See issue #7563 for the original discussion on this matter. If someone is more knowledgable about performance, perhaps we could also add a note that in a TypeScript project, moving the .ts and .tsx extensions to the beginning could be advantageous (per @thymikee).

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[thymikee]

Can we also update the versioned docs with this notice?

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Merging #7616 into master will not change coverage.
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[jeysal]

perhaps we could also add a note that in a TypeScript project, moving the .ts and .tsx extensions to the beginning could be advantageous

Yeah I think something like "We recommend placing the extensions most commonly used in your project on the left" would be good, otherwise it might not be immediately obvious to everyone.

[jeffgore00]

Thanks @jeysal, I like that wording. That's a simple addition for the current version, but for the versioned docs, @thymikee, I think it's potentially confusing for that sentence which says implementation is dependent on your project to co-exist with the command that "If you are using TypeScript this should be ["js", "jsx", "json", "ts", "tsx"].

So for the versioned docs I suggest this rewrite - let me know what you think:

moduleFileExtensions [array]

Default: ["js", "json", "jsx", "node"]

An array of file extensions your modules use. If you require modules without specifying a file extension, these are the extensions Jest will look for, in left-to-right order.

If you are using TypeScript via ts-jest, you will want to add "ts" and/or "tsx" to above default. Where you place these is up to you - we recommend placing the extensions most commonly used in your project on the left.

[SimenB]

@jeffgore00 no reason to bring up "via ts-jest", IMO - just say "If you are using TypeScript, you will...".

Also, could you sign the CLA? 😀

[jeffgore00]

Thanks @SimenB - I signed the CLA and made edits to all of the versioned_docs as well.

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