feat(CI): add the GitHub Super Linter

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

src/string.c

@@ -2570,7 +2570,7 @@ mrb_str_to_dbl(mrb_state *mrb, mrb_value str, mrb_bool badcheck)
  *     str.to_f   => float
  *
  *  Returns the result of interpreting leading characters in <i>str</i> as a
- *  floating point number. Extraneous characters past the end of a valid number
+ *  floating-point number. Extraneous characters past the end of a valid number
  *  are ignored. If there is not a valid number at the start of <i>str</i>,
  *  <code>0.0</code> is returned. This method never raises an exception.
  *