Feature/replace proselint with vale

[chenxsan]

closes #4162

TODO

• replace proselint with vale
• fix some errors in markdown files
• remove Python requirements

Proselint job ran for 2m 46s before according to https://github.com/webpack/webpack.js.org/runs/1436990385?check_suite_focus=true, and now only 45s https://github.com/webpack/webpack.js.org/runs/1437588188?check_suite_focus=true

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[montogeek]

Does it need all this configuration?