More robust spawning of `flatc` with output handling

[atifaziz]

This PR addresses a number of robustness issues with spawning of flatc and harnessing its output. The changes can be summarised as follows:

• Better specification of command-line arguments as a list via ArgumentList so one doesn't have to worry about platform idiosyncrasies with respect to quoting, escaping and separating arguments (these are handled internally by Process).
• Asynchronous reading and collecting of redirected output & error streams to avoid potential for deadlocks.
• Disposing the spawned process via using.

All tests were passing locally with these changes.

[jamescourtney]

Thanks for this! I will review later today!

[codecov[bot]]

Codecov Report

Merging #257 (f263635) into master (0fbcaf7) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
+ Coverage   95.22%   95.25%   +0.02%     
==========================================
  Files         109      109              
  Lines        7147     7168      +21     
  Branches      672      673       +1     
==========================================
+ Hits         6806     6828      +22     
  Misses        237      237              
+ Partials      104      103       -1     

Impacted Files	Coverage Δ
src/FlatSharp.Compiler/FlatSharpCompiler.cs	87.80% <100.00%> (+2.49%)	:arrow_up:

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[jamescourtney]

Thanks for your work here. I'll approve and commit pending this test pass.