Add deprecate() mixin

[MartijnCuppens]

We're probably going to drop a lot of mixins, and maybe some functions in v5. That's why I would like to introduce a deprecate() mixin. This mixin will allow us to deprecate mixins and functions now. Its goal is that all functions/mixins that will be droped in v5 include this mixin so that it'll be easy to check if people are relying on deprecated stuff in their codebase if they want to upgrade from Bootstrap 4 to 5. (They can update to the latest v4 first where all the deprecate() mixins are included.)

I've put #28072, #28067 and #28066 hold for this (I acctually tought $ignore-warning was a global variable, which wasn't the case). Probably going to submit a lot more deprecation PRs (these won't be blocking the 4.3 release)

[mdo]

Should we stick with $enable- here?

Also, this is set to true, but shouldn't it be set fo false to start? This replaces $ignore-warning != true, so it's disabled to start I believe.

[MartijnCuppens]

It doesn't replace $ignore-warning != true. That variable is still used to suppress the warning because .text-hide still uses the mixin:

bootstrap/scss/utilities/_text.scss

Lines 59 to 61 in f7e8445

	.text-hide {
	@include text-hide($ignore-warning: true);
	}

($ignore-warning shouldn't be set here, I've opened #28098 for this) Edit: it's clearer to define the variable, I'll just leave it.

I've changed the variable to an $enable-* variable and set it to false by default. Everything should be clear if you look at the changed code, if not, let me know what need to be clarified more.

[mdo]

Couple copy changes here, but otherwise looking good. Appreciate the mixin approach!