chore: create output directory if not exists

[sssooonnnggg]

If the output directory doesn't exist, the compiler will return directly without any errors, which is a bit confusing.
When the output directory does not exist, try to create it.

[jamescourtney]

Apparently all the pipelines are broken. I'll take a look at this soon.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.91%. Comparing base (cf9cd4a) to head (4453709).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #444   +/-   ##
=======================================
  Coverage   96.91%   96.91%           
=======================================
  Files         126      126           
  Lines        9754     9754           
  Branches      792      792           
=======================================
  Hits         9453     9453           
  Misses        209      209           
  Partials       92       92           

Files with missing lines	Coverage Δ
src/FlatSharp.Compiler/FlatSharpCompiler.cs	93.43% <ø> (ø)

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