Support Unicode colon equals

[ronkok]

This PR adds support for three Unicode characters: ≔ ≕ ⩴

[codecov]

Codecov Report

Merging #1151 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1151      +/-   ##
==========================================
+ Coverage   78.95%   79.01%   +0.06%     
==========================================
  Files          59       59              
  Lines        3872     3875       +3     
  Branches      655      655              
==========================================
+ Hits         3057     3062       +5     
+ Misses        674      672       -2     
  Partials      141      141

Impacted Files	Coverage Δ
src/macros.js	82.6% <100%> (+0.38%)	⬆️
src/functions.js	87.03% <0%> (+3.7%)	⬆️

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[ronkok]

I withheld these characters from PR #933 because three packages address these characters and they all disagree on the alias.

Character	Code Point	mathtools and KaTeX	colonequals and KaTeX	unicode-math
≔	U+2254	\coloneqq	\colonequals	\coloneq
≕	U+2255	\eqqcolon	\equalscolon	\eqcolon
⩴	U+2A74	\Coloneqq	\equalscoloncolon	\Coloneq

Not only that, but the Unicode math aliases are used by mathtools to render a different character.

Alias	unicode-math	mathtools and KaTeX
\coloneq	≔	:−
\eqcolon	≕	−:
\Coloneq	⩴	::−

I have said in earlier PRs that for Unicode support, the internal alias is only a means, not an end. What matters is that the author must get the results desired from the input of a specific Unicode character. That is still true, so I have implemented these Unicode characters by writing macros that call the mathtools aliases.

It’s a good thing that those internal aliases don’t really matter. This could have been ugly.

[ronkok]

Visual confirmation:

[edemaine]

Great work as usual!