feat: use number set to speed up number parsing

[tdakkota]

name                           old time/op    new time/op    delta
Decoder_Float64/floats.json-4    23.4µs ± 2%    21.4µs ± 2%  -8.47%  (p=0.000 n=15+14)

name                           old alloc/op   new alloc/op   delta
Decoder_Float64/floats.json-4     0.00B          0.00B         ~     (all equal)

name                           old allocs/op  new allocs/op  delta
Decoder_Float64/floats.json-4      0.00           0.00         ~     (all equal)

[codecov]

Codecov Report

Merging #40 (75b85e8) into main (fabd6f1) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #40   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           35        35           
  Lines         2309      2307    -2     
=========================================
- Hits          2309      2307    -2     

Impacted Files	Coverage Δ
dec_float.go	100.00% <100.00%> (ø)

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