refactor(toml): make symbolPairs a Map

[timreichen]

Dependent on #6538

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 94.56%. Comparing base (3a021b2) to head (8295176).

Files with missing lines	Patch %	Lines
toml/_parser.ts	80.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6563      +/-   ##
==========================================
- Coverage   94.58%   94.56%   -0.02%     
==========================================
  Files         579      579              
  Lines       43752    43761       +9     
  Branches     6494     6495       +1     
==========================================
+ Hits        41381    41384       +3     
- Misses       2331     2336       +5     
- Partials       40       41       +1     

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