Enable markdownlint rule MD046 code-block-style

Fix C Markdown code block style

Add the rules to the markdownlint config file as comments

build(deps): bump actions/upload-artifact from 2.2.3 to 2.2.4

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 2.2.3 to 2.2.4.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](https://github.com/actions/upload-artifact/compare/v2.2.3...v2.2.4)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  dependency-type: direct:production
  update-type: version-update:semver-patch
...
Signed-off-by: dependabot[bot] <support@github.com>

`mrb_vm_const_get` function looks up the constant first in the base
class, so that fallback `const_get` need not to search from the base.

```ruby
module M
  FOO = 'm'
end
class A
  FOO = 'a'
  prepend M
end
class B < A
  def foo
    p FOO
  end
end
B.new.foo  # should print `m` not `a`
```

Add two more pre-commit hooks

Standardize commands inside backticks

Remove whitespace

Fix shebang by removing whitespace

Enable MD011 no-reversed-links - Reversed link syntax

This reverts commit ee301749.

I misunderstood something and the new behavior was different from CRuby.
The issue was reported by @dearblue, regarding #5478

Add documentation on spell checking with misspell

 Run pre-commit with GitHub Actions

Running pre-commit with GitHub Actions now gives us more tests and coverage

Remove duplicate GitHub Actions for merge conflicts and trailing whitespace

Remove duplicate checks for markdownlint and yamllint from the GitHub Super-Linter

Add new custom pre-commit hook running with a shell script to sort alphabetically and uniquify codespell.txt

Add new pre-commit hook to check spelling with codespell

https://github.com/codespell-project/codespell

Fix spelling

`mrb_float_to_str()` used to take `fmt` argument. We thought no one used
the function, and OK to remove the argument. But at least `mruby-redis`
gem used the function.

Raise `TypeError` with `super` inside `instance_eval` / `class_eval`

Commit d0f60182 introduced an exception as a limitation of mruby.

Subsequent CRuby-2.7 has changed its behavior to raise an exception.
ref: https://github.com/ruby/ruby/commit/55b7ba368696033f2e89b77cbcd4a05dec97b139

* renamed from redundant `readint_mrb_int()`
* supports only base upto 16
* no base validation (already done in parser)
* no negative read (negate after read)
* overflow detection using `mrb_int_{mul,add}_overflow()`

Difference from `strtoul(3)`:

* reads `mrb_int` based on configuration
* specifies the end of the string
* no sign interpretation
* base 10 only

build(deps): bump github/super-linter from 4.2.1 to 4.2.2

Bumps [github/super-linter](https://github.com/github/super-linter) from 4.2.1 to 4.2.2.
- [Release notes](https://github.com/github/super-linter/releases)
- [Commits](https://github.com/github/super-linter/compare/v4.2.1...v4.2.2)

---
updated-dependencies:
- dependency-name: github/super-linter
  dependency-type: direct:production
  update-type: version-update:semver-patch
...
Signed-off-by: dependabot[bot] <support@github.com>