QemuHX: various improvements

[masatake]

• Extract command in { "command", ....
• Implement a selector arbitrating Haxe and QemuHX parsers, both handling
  .hx file extension.
• Recognize SRST keywords.

[codecov]

Codecov Report

❌ Patch coverage is 20.63492% with 50 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.87%. Comparing base (381d57e) to head (8cdde4e).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
optlib/qemuhx.c	0.00%	50 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4382      +/-   ##
==========================================
- Coverage   85.93%   85.87%   -0.07%     
==========================================
  Files         252      252              
  Lines       62715    62774      +59     
==========================================
+ Hits        53895    53906      +11     
- Misses       8820     8868      +48     

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