fix: add whitespaces around operators in arbitrary calc

[schelmo]

twind and tailwind both add spaces around operators when using calc() in square brackets
the regex is borrowed from tw-in-js/twind#176 by @sibbng
hope this is okey for you @sibbng
this fixes #374 and #318

[codecov-commenter]

Codecov Report

Merging #390 (672aa64) into main (728d187) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #390   +/-   ##
=======================================
  Coverage   88.14%   88.14%           
=======================================
  Files          56       56           
  Lines        4773     4775    +2     
  Branches     1024     1025    +1     
=======================================
+ Hits         4207     4209    +2     
  Misses        303      303           
  Partials      263      263           

Impacted Files	Coverage Δ
src/lib/utilities/handler.ts	70.05% <100.00%> (+0.34%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 728d187...672aa64. Read the comment docs.

[sibbng]

Tailwind CSS 2.2 also uses that regex.