Add `Hash#except` [Ruby 3.0]

Add support for squiggly heredocs

Improves out of memory messages for `eval`

The comment says that if `mrb_parse_nstring()` returns `NULL`, it is only out of memory.
I'm worried about compatibility if I set the exception class to `NoMemoryError`, so it's still `RuntimeError`.

Create CODEOWNERS file

Add example for cross-compiling to Nintendo GameBoyAdvance

- You can use a CODEOWNERS file to define individuals or teams that are responsible for code in a repository.
- Code owners are automatically requested for review when someone opens a pull request that modifies code that they own. Code owners are not automatically requested to review draft pull requests.
- https://docs.github.com/en/free-pro-team@latest/github/creating-cloning-and-archiving-repositories/about-code-owners
- This creates a bit more automation so that after you merge this PR, next time someone submits a PR @matz will be automatically assigned as the PR reviewer as CODEOWNER
- CODEOWNERS can even be assigned directories or file types

refactor: fix Rake link and fix missing HTTPS on links

Fix typo in `.github/workflows/build.yml` [skip travis][skip appveyor]

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.