feat: add Dependabot config file

For better coverage.

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

Set for GitHub Actions to check for update everyday.

When you add or update the dependabot.yml file, this triggers an immediate check for version updates. Any options that also affect security updates are used the next time a security alert triggers a pull request for a security update.

https://docs.github.com/en/github/administering-a-repository/configuration-options-for-dependency-updates

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-`.