JS Mangle private properties => reduce minified JS by 11.9% (4.8% gzipped)

[pvdlg]

All the private attribute (variables or function) in Bootstrap JS modules starts with _ so we can easily configure Uglify to mandle them and produce a small minified JS file.

	Minified size	Minified gain	Minified + zip size	Minified + zip gain
Before	46,653b	-	12,648b	-
After	41,059b	11.9%	12,063b	4.8%

In addition, this size reductions cumulate with #21711

[mdo]

This needs review and conflicts resolved before we could merge it now. @Johann-S @XhmikosR can you take a look and let me know if we should proceed with this PR or something else?

[XhmikosR]

@vanduynslagerp: how safe is that?

[XhmikosR]

BTW @vanduynslagerp you would need to move the change in build/uglifyjs.config.json

[pvdlg]

@vanduynslagerp: how safe is that?

It's safe as long as you don't prefix your public class methods and properties with _.
And it provides a benefit as long as you do prefix your private class methods and properties with _.

It was the case when I opened this PR.
I didn't contribute to Bootstrap for a while and I didn't follow the recent updates, so I can't tell about the current safety/efficiency of this modification with the current code base.

Feel free to re-use or adapt the modification or close the PR if the code has changed and doesn't follow the convention of prefixing private methods and properties with _.

[XhmikosR]

@Johann-S: what do you think about this? If you agree we should make a new branch and test this ourselves.

[Johann-S]

Yes sure, our minified file size will grow with the removal off jQuery from our dependencies so optimizations are welcome here 👍