assert: optimize string length calculation in getSimpleDiff

[Ayoub-Mabrouk]

Reduced redundant typeof operations by caching type checks
and simplified the string length calculation using ternary
operators for improved readability.

[BridgeAR]

The calls will likely be inlined during optimization passes in the jit. I am fine with it either way.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/69843/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.56%. Comparing base (2fb82c8) to head (54e8e83).
⚠️ Report is 37 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60331      +/-   ##
==========================================
- Coverage   88.59%   88.56%   -0.03%     
==========================================
  Files         704      704              
  Lines      208474   207800     -674     
  Branches    40067    40035      -32     
==========================================
- Hits       184696   184045     -651     
- Misses      15805    15806       +1     
+ Partials     7973     7949      -24     

Files with missing lines	Coverage Δ
lib/internal/assert/assertion_error.js	95.88% <100.00%> (-0.05%)	⬇️

... and 108 files with indirect coverage changes

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