fix: avoid mutations so lib can be used in a Svelte $derived()

[bartveneman]

closes #28

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.20%. Comparing base (7712027) to head (4ded859).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
- Coverage   98.31%   98.20%   -0.11%     
==========================================
  Files          14       14              
  Lines         950      892      -58     
  Branches      108      106       -2     
==========================================
- Hits          934      876      -58     
  Misses         14       14              
  Partials        2        2              

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[codecov-commenter]

Bundle Report

Changes will decrease total bundle size by 1.87kB (-12.86%) ⬇️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
index.js-esm	12.66kB	-1.87kB (-12.86%) ⬇️

Affected Assets, Files, and Routes:

view changes for bundle: index.js-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
index.js	-1.39kB	10.75kB	-11.47%
index.d.ts	-476 bytes	1.91kB	-19.92%

Files in index.js:

• ./src/lib/index.ts → Total Size: 2.4kB

• ./src/lib/chunkify.ts → Total Size: 1.53kB

• ./src/lib/decuplicate.ts → Total Size: 1.1kB

• ./src/lib/prettify.ts → Total Size: 1.01kB

• ./src/lib/filter-entries.ts → Total Size: 1.01kB

• ./src/lib/extend-ranges.ts → Total Size: 1.03kB