Add purgecss-postcss plugin to remove unused CSS from the bundle

[obulat]

Fixes

Related to #279 by @obulat

Description

Currently, the bundle has a lot of unused CSS from third-party libraries (buefy, bulma) and Vocabulary. To make sure that unused CSS is not packaged into the final build bundle, we remove it with the help of @fullhuman/postcss-purgecss plugin.

Technical details

This PR makes the CSS file ~4 times smaller:

File                                              Size        Gzipped
BEFORE dist\css\app.79796147.css            218.88 KiB      32.01 KiB
AFTER  dist\css\app.f2825e10.css             46.04 KiB       8.89 KiB

Some selectors are whitelisted in the postcss.config.js to make sure that the plugin does not remove necessary styles.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main or master).
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[hugosolar]

nice improvement with postCSS! ⭐