feat(parser): parse negative numbers and exponents

[rahulk29]

Supports parsing numeric literals containing negative numbers and/or
exponents. For example, the SPICE parser now handles "-2", "-2.3",
"-2.34e-19", or "2.34e19".

[codecov]

Codecov Report

Attention: 30 lines in your changes are missing coverage. Please review.

Comparison is base (b74afa1) 40.73% compared to head (829348b) 40.69%.

Files	Patch %	Lines
libs/spice/src/parser/conv.rs	30.23%	6 Missing and 24 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #364      +/-   ##
==========================================
- Coverage   40.73%   40.69%   -0.04%     
==========================================
  Files         167      167              
  Lines       21255    21274      +19     
  Branches     8935     8947      +12     
==========================================
  Hits         8658     8658              
- Misses       6690     6693       +3     
- Partials     5907     5923      +16     

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