encoding/json: optimize isValidNumber

[amarjeetanandsingh]

isValidNumber() performance can be optimized by avoiding substring creation in for loop.

benchstat oldBench.txt newBench.txt 
name             old time/op    new time/op    delta
NumberIsValid-8    39.3ns ± 1%    28.1ns ± 4%  -28.53%  (p=0.000 n=9+10)

name             old alloc/op   new alloc/op   delta
NumberIsValid-8     0.00B          0.00B          ~     (all equal)

name             old allocs/op  new allocs/op  delta
NumberIsValid-8      0.00           0.00          ~     (all equal)

[gopherbot]

Change https://go.dev/cl/426134 mentions this issue: encoding/json: optimize isValidNumber by avoiding substring creation in for loop.

[amarjeetanandsingh]

Duplicate of https://go-review.googlesource.com/c/go/+/425877