lib: optimize TextEncoder.encode for short strings

[mertcanaltin]

This pr improvement short strings for TextEncoder.encode.

Benchmark results:

(.venv) ➜  node git:(mert/textencoder-small-string) ✗ npx node-benchmark-compare te-clean.csv                                                        
                                                                          confidence improvement accuracy (*)   (**)  (***)
util/text-encoder.js op='encode' type='ascii' n=1000000 len=256                    *      1.67 %       ±1.45% ±1.94% ±2.54%
util/text-encoder.js op='encode' type='ascii' n=1000000 len=32                   ***    127.18 %       ±1.93% ±2.58% ±3.40%
util/text-encoder.js op='encode' type='one-byte-string' n=1000000 len=256                 1.39 %       ±2.07% ±2.79% ±3.68%
util/text-encoder.js op='encode' type='one-byte-string' n=1000000 len=32         ***     85.44 %       ±2.43% ±3.27% ±4.31%
util/text-encoder.js op='encode' type='two-byte-string' n=1000000 len=256                -0.12 %       ±0.86% ±1.15% ±1.49%
util/text-encoder.js op='encode' type='two-byte-string' n=1000000 len=32         ***     74.83 %       ±1.45% ±1.94% ±2.54%

Be aware that when doing many comparisons the risk of a false-positive result increases.
In this case, there are 6 comparisons, you can thus expect the following amount of false-positive results:
  0.30 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.06 false positives, when considering a   1% risk acceptance (**, ***),
  0.01 false positives, when considering a 0.1% risk acceptance (***)

I got help from Claude Opus 5.5. I reviewed the changes and ran the tests and benchmarks myself.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.49%. Comparing base (e2d5522) to head (5f342bd).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66831      +/-   ##
==========================================
- Coverage   90.50%   90.49%   -0.01%     
==========================================
  Files         797      797              
  Lines      277793   277809      +16     
  Branches    53366    53366              
==========================================
+ Hits       251405   251414       +9     
- Misses      16801    16806       +5     
- Partials     9587     9589       +2     

Files with missing lines	Coverage Δ
lib/internal/encoding.js	96.74% <100.00%> (+0.08%)	⬆️

... and 33 files with indirect coverage changes

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