All: Mark positional selectors as deprecated since jQuery 3.4.0

[mgol]

Positional selectors: :first, :last, :eq, :even, :odd, :lt, :gt, and :nth are deprecated since jQuery 3.4.0. This is now reflected in the docs & most deprecated pseudo-classes are not used in examples on other API pages.

The only exception is :even & :odd which are still used on some pages. I wasn't sure whether I should migrate them to a custom implementation using .filter() or whether we want .even() & .odd() in Core. I did a quick implementation to check the size impact & that would add 25 bytes to the gzipped file.

Fixes gh-1144

[dmethvin]

Looks like you caught all the usage, thanks! I don't know how common the even/odd usage is. We should put it into Migrate for sure. If we don't put it in core we could just mention the simple implementation of each in the Migrate warning and people can add that to their code.

[mgol]

This is the implementation that adds +25 bytes: mgol/jquery@f454097

	even: function() {
		return this.filter( function( i ) {
			return ( i + 1 ) % 2;
		} );
	},

	odd: function() {
		return this.filter( function( i ) {
			return i % 2;
		} );
	},

[mgol]

OK, I'm going to merge it; we can discuss the .even()/.odd() issue separately.

[mgol]

I extracted the :even/:odd issue to #1147.