Grammars for ES2015-ES2022

[elgertam]

I have added preliminary grammars for all JavaScript releases from ES2015 through ES2022

• generate-ecmascript-grammar.mjs and grammarkdown.ohm have been updated to parse later ES specs
• extract-grammarkdown.mjs and dedent.py have been included to create a clean Grammarkdown grammar from an ES spec
• Extracted & manually cleaned ES grammars for each release
• Per-grammar overrides have been moved from generate-ecmascript-grammar.mjs to an override.json file, along with productions containing reserved words
• Grammarkdown grammars & overrides have been organized into folders for each version of ES
• Generated Ohm grammars for each ES version
  • Grammars appear to parse JS programs correctly in the Ohm editor
  • Using the esm module loader syntax is only supported if the start symbol is changed to Module

[pdubroy]

I've merged into a branch for now, so we can both collaborate on that: https://github.com/ohmjs/ohm/tree/elgertam-ecmascript