feat: add is_custm(str: string): boolean helper

[bartveneman]

closes #85

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.67%. Comparing base (757909c) to head (4a33497).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #87   +/-   ##
=======================================
  Coverage   94.66%   94.67%           
=======================================
  Files          15       15           
  Lines        2362     2365    +3     
  Branches      590      592    +2     
=======================================
+ Hits         2236     2239    +3     
  Misses        126      126           

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[codecov-commenter]

Bundle Report

Changes will increase total bundle size by 278 bytes (0.22%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
@projectwallace/css-parser-esm	127.2kB	278 bytes (0.22%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: @projectwallace/css-parser-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
string-*.js	168 bytes	3.27kB	5.41% ⚠️
index.js	110 bytes	1.78kB	6.59% ⚠️

Files in string-*.js:

• ./src/string-utils.ts → Total Size: 2.85kB

Files in index.js:

• ./src/index.ts → Total Size: 0 bytes