Use `sizeof` rather than `length` for file length check

[MichaelHatherly]

This issue only seems to occur when there is a file with contents that contained just enough Unicode characters to push the sizeof value over 254 while keeping length under that limit. I'd assume that isfile really actually cares about number of bytes rather than apparent character length.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (0dd0f0f) 89.69% compared to head (f0df3c6) 89.69%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #260 +/- ## ======================================= Coverage 89.69% 89.69% ======================================= Files 10 10 Lines 1775 1775 ======================================= Hits 1592 1592 Misses 183 183 ``` | [Impacted Files](https://app.codecov.io/gh/quinnj/JSON3.jl/pull/260?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jacob+Quinn) | Coverage Δ | | |---|---|---| | [src/utils.jl](https://app.codecov.io/gh/quinnj/JSON3.jl/pull/260?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jacob+Quinn#diff-c3JjL3V0aWxzLmps) | `89.47% <100.00%> (ø)` | |

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