Provide named character entities for invisible and ambiguous Unicode characters about html HOT 3 OPEN

Can this be folded into #5121 or vice versa? I'm not sure why we need two issues for this.

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I get that &6msp; for the SIX-PER-EM SPACE might be derived from a standard Unicode abbreviation (couldn't actually find the relevant standard at the moment) but given that THREE-PER-EM SPACE and FOUR-PER-EM SPACE are already   and  , shouldn't this one be &emsp16; for consistency?
The way it is right now seems confusing.

Similarly, it might make sense to change &nqsp; and &mqsp; to &enqsp; and &emqsp; for consistency with the other em/en related ones as well.

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Thank @r12a for bringing this up.

It is very welcome to use named entities instead of the digits of the codepoint and their markup syntax x#&HHHH;.

About formatting characters, one can perhaps remember the very common [202A/202B, 202D] to delimit bidi sentences (although you still have to remember that A is for left and B is for right...) But now there are the others: lro/rlo coded 202D/202E, and rli/lri fsi pdfi coded coded 2066/2067 2068 2069, not the same range...

Especially since some HTML editors also replace the digital entities x#&HHHH; by the corresponding Unicode characters U+HHHH which are invisible in the source. Whereas if it's a named entity they put a visible mark instead.

(BTW, I've always wondered why ‎/‏ and not &lre; etc.?)

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