parser: allow changing oscode mapping variant at runtime

[rszyma]

Describe your changes. Use imperative present tense.

Add a global variable OSCODE_MAPPING_VARIANT that can be manually set to use other OsCode mapping variant.
This change will allow me to fix rszyma/vscode-kanata#5.

Checklist

• Add documentation to docs/config.adoc
  • N/A
• Add example and basic docs to cfg_samples/kanata.kbd
  • N/A
• Update error messages
  • N/A
• Added tests, or did manual testing
  • No need for testing