lib: use bracket notation instead of startsWith/endsWith for single char

[kimtaejin3]

This PR avoids potential prototype pollution and improves performance by replacing method calls with
direct property access.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.78%. Comparing base (5d39030) to head (d6f31dc).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61500      +/-   ##
==========================================
- Coverage   89.82%   89.78%   -0.04%     
==========================================
  Files         667      672       +5     
  Lines      203693   203907     +214     
  Branches    39163    39199      +36     
==========================================
+ Hits       182963   183078     +115     
- Misses      13061    13162     +101     
+ Partials     7669     7667       -2     

Files with missing lines	Coverage Δ
lib/internal/socketaddress.js	98.98% <100.00%> (ø)

... and 48 files with indirect coverage changes

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