Better perf script format detection

[mstange]

The perf script format is a text format that doesn't contain a distinctive header. Our detection was too general and was incorrectly detecting JSON profiles as perf script profiles.

Now we bail out if the file starts with an opening brace.

I've also simplified how we treat perf script profiles with header. Rather than skipping the header and detecting the first sample, we just detect the header.

[codecov[bot]]

Codecov Report

Merging #4187 (ff28d27) into main (b7eba13) will increase coverage by 0.00%. The diff coverage is 100.00%.

:exclamation: Current head ff28d27 differs from pull request most recent head 6443ace. Consider uploading reports for the commit 6443ace to get more accurate results

@@           Coverage Diff           @@
##             main    #4187   +/-   ##
=======================================
  Coverage   88.45%   88.46%           
=======================================
  Files         280      280           
  Lines       24528    24527    -1     
  Branches     6535     6536    +1     
=======================================
  Hits        21697    21697           
+ Misses       2629     2628    -1     
  Partials      202      202           

Impacted Files	Coverage Δ
src/profile-logic/import/linux-perf.js	96.46% <100.00%> (+0.84%)	:arrow_up:

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