CI: add textlint

Instead of requesting changes in every pull requests and block them from merging,
we are trying to automate the process of adding missing spaces between half and
full-width characters using GitHub Actions.

The main idea is:

- Only run the workflow in the main repository (instead of other forks).
- Try to fix all files using textlint.
- Only commit files with .md extension.
- Directly push the change to the master branch.

Related projects:

- https://github.com/textlint/textlint
- https://github.com/textlint-ja/textlint-rule-preset-ja-spacing
- https://github.com/sparanoid/chinese-copywriting-guidelines

Related discussions:

- https://github.com/Anduin2017/HowToCook/pull/88