Otherwise we suffer `(unknown):0:` errors.

On configurations where `sizeof(mrb_int) > sizeof(int)`.

`MRuby::Presym` no longer needs `Rake::DSL`

The GitHub Super Linter is a more robust and better supported
tool than the current GitHub Actions we are using.

Running these checks:

ERROR_ON_MISSING_EXEC_BIT: true
VALIDATE_BASH: true
VALIDATE_BASH_EXEC: true
VALIDATE_EDITORCONFIG: true
VALIDATE_MARKDOWN: true
VALIDATE_SHELL_SHFMT: true
VALIDATE_YAML: true

https://github.com/marketplace/actions/super-linter
https://github.com/github/super-linter

Added the GitHub Super Linter badge to the README.

Also updated the pre-commit framework and added
more documentation on pre-commit.

Added one more pre-commit check: check-executables-have-shebangs

Added one extra check for merge conflicts to our
GitHub Actions.

EditorConfig and Markdown linting.

Minor grammar and spelling fixes.

Update linter.yml

Fix `enable_debug_info?` in `mrbgems/mruby-proc-ext/test/proc.rb`

Fix incorrect prototype on declaration of mrb_protect_atexit

build(deps): bump actions/cache from v2.1.4 to v2.1.5

`state.c` makes a prototype declaration for the private
`mrb_protect_atexit` which is defined in `error.c`. `error.c` defines
this function with a void return type, but `state.c` defines the
prototype with an `int` return type.

This mismatch prevents mruby from compiling on stricter compilers like
emscripten.

Bumps [actions/cache](https://github.com/actions/cache) from v2.1.4 to v2.1.5.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](https://github.com/actions/cache/compare/v2.1.4...1a9e2138d905efd099035b49d8b7a3888c653ca8)
Signed-off-by: dependabot[bot] <support@github.com>

- remove `Integer#chr` (thus `mruby-sting-ext`) dependency
- fix the behavior when `c.is_a? String`
- fix the behavior when `c > 255`

This CI could consume too much CPU time on GitLab. Maybe we should add
resource concious CI configuration on GitLab.

chore: fix spelling

Reorganize `mcall()` in `mruby-method`

According to CodeQL instruction, `HEAD^2` is no longer needed.

build(deps): bump actions/upload-artifact from v2.2.2 to v2.2.3

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from v2.2.2 to v2.2.3.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](https://github.com/actions/upload-artifact/compare/v2.2.2...ee69f02b3dfdecd58bb31b4d133da38ba6fe3700)
Signed-off-by: dependabot[bot] <support@github.com>

Updating dreamcast_shelf build configuration.

* `--cc` print compiler name
* `--ld` print linker name

Fix heading level in `doc/guides/link.md` [ci skip]

Fix typo in `doc/guides/link.md` [ci skip]