fix(multer): export Multer class contract for consumption

[peterblazejewicz]

This tries to fix issue with non-exported interface of the multer class
to the creation code itself, which prevents from using instance methods
in proper way.
See #41970 for discussion of the problem.

/cc @restjohn

Thanks!

Fixes #41970

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Add or edit tests to reflect the change. (Run with npm test.)

[peterblazejewicz]

I looks change is backward ccompatible. Could be useful if someone with more multer usage logged can look, thanks!

[typescript-bot]

@peterblazejewicz Thank you for submitting this PR!

This is a live comment which I will keep updated.

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Because you edited one package and updated the tests (👏), I can help you merge this PR once someone else signs off on it.

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---

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[typescript-bot]

🔔 @jt000 @vilic @DavidBR-SW @mxl @HyunSeob @PierreTchuente @thrymgjol — please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

[typescript-bot]

👋 Hi there! I’ve run some quick measurements against master and your PR. These metrics should help the humans reviewing this PR gauge whether it might negatively affect compile times or editor responsiveness for users who install these typings.

Let’s review the numbers, shall we?

Comparison details 📊

	master	#46330	diff
Batch compilation
Memory usage (MiB)	69.3	65.7	-5.2%
Type count	10708	10708	0%
Assignability cache size	1987	1987	0%
Language service
Samples taken	109	111	+2%
Identifiers in tests	109	111	+2%
getCompletionsAtPosition
Mean duration (ms)	352.3	356.2	+1.1%
Mean CV	10.7%	11.3%
Worst duration (ms)	573.6	585.3	+2.0%
Worst identifier	upload	upload
getQuickInfoAtPosition
Mean duration (ms)	353.0	355.0	+0.6%
Mean CV	10.5%	10.5%	-0.6%
Worst duration (ms)	593.1	592.9	0.0%
Worst identifier	post	post

It looks like nothing changed too much. I won’t post performance data again unless it gets worse.

[restjohn]

Nice @peterblazejewicz, thanks! I'm just curious, why an abstract class vs. an interface?

[peterblazejewicz]

I'm just curious, why an abstract class vs. an interface?

No specific reason, IMO could be interface to be more slick.
Alas, could be more sense to have that as interface though (the module does not export the class truly,
let me redo that detail as second thought).

[peterblazejewicz]

@restjohn I've redone this detail, thanks for commenting!