Propagate isJS to symbolicated funcs

[mstange]

I'm currently testing a mode in samply where native symbolication supplies the name for JS functions. This means that we now have frames which are both "JS" and "native" at the same time - they should be visible in the JS-only call tree, but they also have a native library and an address and can get their names by symbolicating an address.

One thing I noticed is that, when symbolication has to add new funcs to the func table, these funcs always had isJS == false. But when symbolicating JS frames via native symbolication, we actually want to propagate isJS from the unsymbolicated function.

Here's a badly-symbolicated example: https://share.firefox.dev/4d0sYMp
The function intersect disappears in JS-only mode.

(The above profile also demonstrates inline symbolication but I realize now that the actual bug doesn't require inlining at all.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.44%. Comparing base (cb9c976) to head (e4ef68b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5907      +/-   ##
==========================================
+ Coverage   85.42%   85.44%   +0.02%     
==========================================
  Files         321      321              
  Lines       32168    32170       +2     
  Branches     8785     8869      +84     
==========================================
+ Hits        27479    27489      +10     
+ Misses       4253     4245       -8     
  Partials      436      436              

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