Use `git ls-files` instead of `find` in `spell-checker.yml` [ci skip]

To avoid including `.git` directory.

Reorganize GitHub Actions configuration [skip travis][skip appveyor]

refactor: fix case of GitHub and fix missing HTTPS on link

* Skip if commit message contains `ci skip`, `skip ci`, or `skip gha`
  ENCLOSED WITH BRACKETS (excluding lint job).
* Separate build and test. This is because builds can be run in parallel,
  but running tests in parallel can cause logs to get mixed up or not
  finished.
* Don't use Chocolatey because it seems to take 1-2 minutes to start up.
* Use the cache better.
* Use `actions/checkout@v2` instead of `actions/checkout@v1`.
* Remove unnecessary package installation.
* Remove unnecessary flag settings.
* Remove `-j` for rake because it doesn't seem to have any effect.
* Rename `main.yml` to `oss-fuzz.yml` to clarify.

- changed `github` to `GitHub`

refactor: remove trailing whitespace

Add my name to the AUTHORS file

feat(CI): add a GitHub Action to lint the Markdown

Ignore `mruby-bin-config` in cross-building

Run on pull request only
Using https://www.npmjs.com/package/markdownlint-cli
Lint Markdown for rules:
- MD009/no-trailing-spaces
- MD012/no-multiple-blanks
- MD022/blanks-around-headings
- MD031/blanks-around-fences
- MD032/blanks-around-lists

🔒 Fix missing HTTPS on links

🔒 Fix missing HTTPS on links

This is because `MRuby::Build.currrent` and `MRuby::CrossBuild.current` are assignments to different instance variables.

feat(CI): add a GitHub Action to check spelling

- Fix spelling
- Run only on pull request
- Using https://github.com/client9/misspell

feat(CI): add a GitHub Action to lint the YAML

Rename build configuration files for CI

Currently, there are build configuration files for CI, `travis.rb` and
`appveyor.rb`, but they are used for GCC/Clang and MSVC, not for Travis CI
and Appveyor, respectively.

Therefore, rename them to `gcc-clang.rb` and `msvc.rb`, respectively, and
move them under `build_config/ci/` to clarify that they are for CI.

Guess toolchain when `MRuby::Build#toolchain` argument is omitted

- Run only on pull request
- Add a `.yamllint` config file
- Lint YAML

Ensure initialization of `RVALUE_zero` in `mrb_obj_alloc`

Output build log at the time of installation

refactor: remove trailing whitespace from C, Header, Ruby and YAML files

Lint

Union initialization initializes the first member. The first member of
`RVALUE` is `struct free_obj`, but because it is only 4-words, it seems that
initialization after the 5th word is not ensured.

Therefore, I created 6-words `struct RVALUE_initializer` for initialization
and made it the first member.

Fix spelling

```ruby
def method_missing(meth, ...)
  send(:"do_#{meth}", ...)
end
```

They used to be `size_t`, `uint32_t` and `ptrdiff_t`. Now all of them
made to be `uint32_t`.