Refine checking for trailing whitespace [skip travis][skip appveyor]

Remove trailing tab in `build_config/dreamcast_shelf.rb` [ci skip]

Introduce EditorConfig [skip ci]

Place optable in ROM

* Include tabs in checking.
* Use `git grep` to avoid including `.git` directory.
* Avoid running `grep` multiple times.

refactor: add the spell checker Action to the Lint Action file

Order jobs in workflow.

chore: fix missing HTTPS on links

chore: fix spelling

Normally a single spell checker can't find all the mistakes or check all types of code.

These mistakes were found by another spell checker inside my editor with a more manual sift / find.

feat(CI): check for trailing whitespace

Run on pull request only.
Use a shell script to check for trailing whitespace in all files.
Fail if trailing whitespace is found.

refactor: only use certified GitHub Actions to increase security

Official -> "The individual jobs in a workflow can interact with (and compromise) other jobs. For example, a job querying the environment variables used by a later job, writing files to a shared directory that a later job processes, or even more directly by interacting with the Docker socket and inspecting other running containers and executing commands in them.

This means that a compromise of a single action within a workflow can be very significant, as that compromised action would have access to all secrets configured on your repository, and can use the GITHUB_TOKEN to write to the repository. Consequently, there is significant risk in sourcing actions from third-party repositories on GitHub. "

https://docs.github.com/en/actions/learn-github-actions/security-hardening-for-github-actions#using-third-party-actions

Update compile prerequisites in `doc/guides/compile.md` [ci skip]

Check if `esclen` is smaller than `len` (original string length).

Bison and gperf are optional.

- Remove Ubuntu-16.04
- Add Ubuntu-20.04 {gcc,clang}

Fix build `mruby-catch` gem without presym

Add `UncaughtThrowError#{tag,value}` for Ruby compatibility

Remove "skip ci" hack to GitHub Actions [skip travis][skip appveyor]

GitHub Actions now supports "skip ci" natively:
https://github.blog/changelog/2021-02-08-github-actions-skip-pull-request-and-push-workflows-with-skip-ci/

It used to return `mrb_ssize` but its size may differ from `mrb_int`,
e.g. `MRB_INT64` on `MRB_32BIT` architecture.

Merge pull request #5334 from shuujii/avoid-possibility-of-reading-uninitialized-areas-in-h_check_modified

Avoid possibility of reading uninitialized areas in `h_check_modified()`

In `h_check_modified()`, in the case of `MRB_NO_BOXING`, `ht_ea()` or
`ht_ea_capa()` for AR may read uninitialized area. Therefore, do not use
those macros for AR in `MRB_NO_BOXING` (but in the case of `MRB_64BIT`,
`ht_ea_capa()` is the same as `ar_ea_capa()`, so use it).

fix #5332

Binary gems description added, along with a few cosmetic changes.

Binary gems are mrbgems that set `spec.bins` in their `mrbgem.rake`,
and usually their names are prefixed with `mruby-bin-`.

Especially description regarding the build process and `build` directory
structures, along with some typo fixes and cosmetic changes.

Fix heap-buffer-overflow for small `Hash` (HT) in `Hash#rehash`