Enhance font_range to check for emoji composition triggers

Enhance font_range to check for emoji composition triggers

If the codepoint that triggered composition is from the emoji script,
use the emoji font to check the string being composed, rather than the
font of the first character of the string.  This makes e.g.

"emoji codepoint with Emoji_Presentation = No followed by VS-16 (FE0F)"

display the emoji version of the glyph for that codepoint.

* admin/unidata/blocks.awk: Add VS-1 through VS-16 to the emoji
script.
* src/composite.c (autocmp_chars): Accept additional argument CH for
the codepoint that triggered composition, pass it to font_range.
(composition_reseat_it, find_automatic_composition): Pass codepoint
that triggered composition to autocmp_chars.
* src/font.c (font_range): Accept additional argument CH for the
triggering codepoint.  If the codepoint is from the 'emoji' script,
use Vscript_representative_chars to find the font to use for the
composition attempt.
(syms_of_font): Add Qemoji symbol.
* src/font.h: Update font_range prototype for argument CH.
* etc/NEWS: Announce change.