Add CI/CD to publish gem

[kachick]

2a302b28c374a8c8da81f796225b1a3a8398d37cf04aacfc0553cf70029bf4e1  ./irb-power_assert.gem
Unpacked gem: '/home/runner/work/irb-power_assert/irb-power_assert/pkg/irb-power_assert'
.
└── irb-power_assert
    ├── LICENSE.txt
    ├── README.md
    └── lib
        ├── irb
        │   ├── cmd
        │   │   └── pa.rb
        │   ├── power_assert
        │   │   └── version.rb
        │   └── power_assert.rb
        └── irb-power_assert.rb

5 directories, 6 files

[kachick]

Better than rake build for this purpose, I think