feat(mf-parser): allow decimal isotopic composition

[lpatiny]

closes: #252

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.55%. Comparing base (4dddbfb) to head (dd51cc1).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/mf-parser/src/parse.js	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #253   +/-   ##
=======================================
  Coverage   96.55%   96.55%           
=======================================
  Files         213      213           
  Lines       19776    19776           
  Branches     2243     2245    +2     
=======================================
  Hits        19094    19094           
  Misses        674      674           
  Partials        8        8           

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