update installation section to refer Vim's official package document

fix CI status badge in readme

fix test case descriptions

add tests for markers consisting of more than 7 characters

fix comma is always prepended to `b:match_words`

fix tests broken by #25

revert 0e9686f without using `,` in marker patterns to avoid matchit.vim bug

Revert "Merge pull request #25 from sybereal/fix/support-conflictmarkersize"

Merge pull request #25 from sybereal/fix/support-conflictmarkersize

install setuptools explicitly for legacy package

fix invalid input for `actions/setup-python`

fix CI workflow

Merge pull request #27 from kyoh86/doc-mapping

Add documents for mappings

Update README for conflict-marker-size > 7