fix: parse short option with trailing equals and empty value (-o=) as empty string

[youdie006]

Fixes #367

Problem

A short option written with a trailing = and no value mis-parses. The = is treated as a standalone key and the option becomes boolean true:

parser(['-o='], { alias: { o: 'opt' } })
// actual:   { _: [], o: true, opt: true, '=': true }
// expected: { _: [], o: '', opt: '' }

The equivalent long option already behaves correctly, which is the tell that this is a short-option-only bug:

parser(['--opt='])  // => { _: [], opt: '' }   (correct)

Root cause

In the short-option branch of lib/yargs-parser.ts, letters = arg.slice(1, -1).split('') strips the trailing =. For -o= that leaves letters = ['o'], so the letters[j + 1] === '=' guard never matches. Parsing falls through to setArg('o', defaultValue('o')) (boolean true) and then key = arg.slice(-1)[0] picks the trailing = up as a spurious key.

Fix

next is already computed as arg.slice(j + 2) and equals exactly '=' only on the final letter when the arg ends in a trailing =. Adding that as an alternative trigger routes -o= through the existing =-handling branch, where value = arg.slice(j + 3) correctly resolves to '':

if ((letters[j + 1] && letters[j + 1] === '=') || next === '=') {

Non-empty values (-o=x) are unaffected -- they still match the original letters[j + 1] === '=' condition.

Tests

Added a case next to the existing empty-value short-option test covering both -o= (empty) and the -o=x control. Full suite: 363 passing, lint clean.

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This change was prepared with AI assistance and reviewed by the author.