Fix JSON3.write for non-traditional indexed AbstractVector

[quinnj]

The previous code was assuming that length(x) was comparable to each eachindex(x), which isn't always true for weird non-linear arrays.

[codecov]

Codecov Report

Patch coverage: 83.33% and no project coverage change.

Comparison is base (b23f72b) 89.69% compared to head (9c3e910) 89.69%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #261   +/-   ##
=======================================
  Coverage   89.69%   89.69%           
=======================================
  Files          10       10           
  Lines        1775     1775           
=======================================
  Hits         1592     1592           
  Misses        183      183           

Impacted Files	Coverage Δ
src/write.jl	84.68% <83.33%> (ø)

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