Fix text truncation for frames named after Object.prototype methods

[canova]

Fixes#6006.

TextMeasurement's width cache was a plain object, so looking up _cache["toString"] returned Object.prototype.toString instead of undefined. getFittedText treated that function as a cached width, which collapsed the truncation algorithm and produced just "…".

This affected JS frames whose names collide with Object.prototype: toString, hasOwnProperty, valueOf, constructor, isPrototypeOf, propertyIsEnumerable, toLocaleString, proto. Visible in the flame graph, stack chart, marker chart, and js-tracer canvases.

Switching the cache to a Map solves this problem.

Example profile:
Before / After

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.77%. Comparing base (3182970) to head (dc80476).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #6044 +/- ##
==========================================
- Coverage 83.78% 83.77% -0.01% 
==========================================
Files 329 329 Lines 34528 34546 +18 Branches 9659 9666 +7 ==========================================
+ Hits 28930 28942 +12 - Misses 5169 5175 +6 
Partials 429 429 

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[fatadel]

LGTM! This looks as easy to unit test, however, I could not find any. For example, smth like getTextWidth("toString") would be sufficient I guess.

[canova]

Yeah, I wanted to add a test first too, but couldn't find any test to piggyback off of. Then I let claude write some and I didn't like because it was too long 😄 I think it's fine as this is very niche.