chore: improve bundle size

[layershifter]

This PR improves package's bundle size:

• replace classnames with clsx (like in Material UI [styles] Replace classnames with clsx mui/material-ui#14152). classnames still don't have ES modules build 😭
• use loose mode for @babel/plugin-proposal-class-properties & @babel/env to simplify transforms

[github-actions]

size-limit report

Path	Size
bundle-size/dist/Button.size.js	67.39 KB (-5.13% 🔽)
bundle-size/dist/Icon.size.js	30.58 KB (-9.27% 🔽)
bundle-size/dist/Image.size.js	62.64 KB (-5.57% 🔽)
bundle-size/dist/Modal.size.js	104.82 KB (-3.64% 🔽)
bundle-size/dist/Portal.size.js	48.75 KB (-2.37% 🔽)

[codecov-commenter]

Codecov Report

Merging #4011 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4011   +/-   ##
=======================================
  Coverage   99.84%   99.84%           
=======================================
  Files         185      185           
  Lines        3251     3251           
=======================================
  Hits         3246     3246           
  Misses          5        5           

Impacted Files	Coverage Δ
src/collections/Breadcrumb/Breadcrumb.js	100.00% <ø> (ø)
src/collections/Breadcrumb/BreadcrumbDivider.js	100.00% <ø> (ø)
src/collections/Breadcrumb/BreadcrumbSection.js	100.00% <ø> (ø)
src/collections/Form/Form.js	100.00% <ø> (ø)
src/collections/Form/FormField.js	100.00% <ø> (ø)
src/collections/Form/FormGroup.js	100.00% <ø> (ø)
src/collections/Grid/Grid.js	100.00% <ø> (ø)
src/collections/Grid/GridColumn.js	100.00% <ø> (ø)
src/collections/Grid/GridRow.js	100.00% <ø> (ø)
src/collections/Menu/Menu.js	100.00% <ø> (ø)
... and 129 more

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