bug#54562: 28.0.91; Emoji sequence not composed

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 25 Mar 2022 17:17:26 +0800
> From:  Po Lu via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> The following Emoji does not display correctly:
> 
>   7⃣️
> 
> In other programs, it displays as the digit "7" inside a square, but
> inside Emacs it displays as the digit "7", followed by the blue square,
> and an empty hollow black square.

I think this means your default font doesn't support the U+20E3
COMBINING ENCLOSING KEYCAP character.  Emacs cannot compose characters
that aren't supported by the font used for the base character.  Here's
what I see in "C-u C-x =" on my system, when Emacs uses a font that
does support it (and where I do see "7" inside a square):

	       position: 148 of 150 (98%), column: 2
	      character: 7 (displayed as 7) (codepoint 55, #o67, #x37)
		charset: ascii (ASCII (ISO646 IRV))
  code point in charset: 0x37
		 script: latin
		 syntax: w 	which means: word
	       category: .:Base, a:ASCII, l:Latin, r:Roman
	       to input: type "C-x 8 RET 37" or "C-x 8 RET DIGIT SEVEN"
	    buffer code: #x37
	      file code: #x37 (encoded by coding system iso-latin-1-dos)
		display: composed to form "7⃣️" (see below)

  Composed with the following character(s) "⃣️" using this font:
    harfbuzz:-outline-Symbola-normal-normal-normal-serif-16-*-*-*-p-*-iso8859-1
  by these glyphs:
    [0 2 55 26 8 0 7 11 0 nil]
    [0 2 8419 2327 0 -10 4 10 4 nil]
    [0 2 65039 3 4 0 1 0 1 [0 0 0]]
  with these character(s):
    ⃣ (#x20e3) COMBINING ENCLOSING KEYCAP
    ️ (#xfe0f) VARIATION SELECTOR-16