Fix short option with an embedded = later in the argument

[shadowspawn]

Problem

A short option with an embedded = later in the argument is parsed inconsistently and with data loss.

In particular:

-a=b=c gives same result as -a=b (lost trailing =c)

-ab=c gives same result as -a=c (lost leading b)

Fixes #5

Solution

Make code more explicit and test for = at start when looking for possible value.

-a=b=c makes option a have value b=c.

-ab=c makes option a true and option b have value c.

(In my first commit I made -ab=c give option b the value =c by only treating the = as special at the start. I checked Yargs and found it set value to c. I can see a rationale for the parsing to treat it like an expansion, so -ab=c => -a -b=c, and option b has value c. More comfortable matching Yargs with this behaviour.)

[ljharb]

Seems like a clear semver-patch, thanks

[codecov-commenter]

Codecov Report

Merging #6 (b62a195) into main (3124ed3) will not change coverage.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main       #6   +/-   ##
=======================================
  Coverage   98.75%   98.75%           
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  Files           1        1           
  Lines         161      161           
  Branches       71       71           
=======================================
  Hits          159      159           
  Misses          2        2           

Impacted Files	Coverage Δ
index.js	98.75% <100.00%> (ø)

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