Avoid warnings with `ruby -cw`

```console
% for rb in `git ls-files '*/mrblib/*.rb' 'mrblib'`; do ruby30 -cw $rb > /dev/null; done
mrbgems/mruby-array-ext/mrblib/array.rb:389: warning: assigned but unused variable - ary
mrbgems/mruby-array-ext/mrblib/array.rb:663: warning: assigned but unused variable - len
mrbgems/mruby-hash-ext/mrblib/hash.rb:119: warning: possibly useless use of a variable in void context
mrbgems/mruby-hash-ext/mrblib/hash.rb:259: warning: assigned but unused variable - keys
mrbgems/mruby-io/mrblib/io.rb:229: warning: literal in condition
mrbgems/mruby-io/mrblib/io.rb:280: warning: literal in condition
mrbgems/mruby-string-ext/mrblib/string.rb:347: warning: assigned but unused variable - len
mrbgems/mruby-toplevel-ext/mrblib/toplevel.rb:2: warning: parentheses after method name is interpreted as an argument list, not a decomposed argument
```

Fixed finding variables from `proc` in `binding.eval` failed

Fixed finding variables defined in the upper proc failed

Previously the following code did not produce the expected results:
```ruby
bx = binding
block = bx.eval("a = 1; proc { a }")
bx.eval("a = 2")
p block.call  # Expect 2 but return 1 due to a bug
```

The previous implementation of `Binding#eval` evaluated the code and then merged the top layer variables.
This patch will parse and expand the variable space before making a call to `eval`.
This means that the call to `Binding#eval` will do the parsing twice.

In addition, the following changes will be made:
- Make `mrb_parser_foreach_top_variable()`, `mrb_binding_extract_proc()` and `mrb_binding_extract_env()` functions private global functions.
- Remove the `posthook` argument from `mrb_exec_irep()`.
  The `posthook` argument was introduced to implement the `binding` method.
  This patch is unnecessary because it uses a different implementation method.

ref #5362
fixed #5491

Fix memory leak in `Kernel#eval`

If no new variable was defined in the `eval` method, the variable was hidden from the nested `eval` method.

```ruby
a = 1
p eval %(b = 2; eval %(a)) # => 1 (good)
p eval %(eval %(a)) # => undefined method 'a' (NoMethodError)
```

This issue has occurred since mruby 3.0.0.

The `mrbc_context` remained unreleased when the `mrb_parse_nstring()` function returned `NULL`.

- `Module#method_added`
- `BasicObject#singleton_method_added`

Add official pre-commit file-contents-sorter

Remove unneeded bash script sort-codespell-wordlist.sh

https://github.com/pre-commit/pre-commit-hooks#file-contents-sorter

* android_arm64-v8a.rb -> android_arm64_v8a.rb
* no-float.rb -> host-nofloat.rb

Enable markdownlint rules MD003,MD005,MD007

pre-commit autoupdate

https://pre-commit.com/#pre-commit-autoupdate

Lint Markdown

https://github.com/DavidAnson/markdownlint#rules--aliases

- `truncate`
- `floor`
- `ceil`

`round` already takes `ndigits`.

Added `MRB_OBJ_ALLOC()` macro that does not require a cast

The `MRB_OBJ_ALLOC()` macro function returns a pointer of the type corresponding to the constant literal defined in `enum mrb_vtype`.

- ` mrb_block_given_p()` -- The name comes from CRuby's `rb_block_given_p ()`

At the same time, it applies to `f_instance_eval()` and `f_class_eval()` of `mruby-eval`.

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