Changes in this PR

• The job update-schema is added to Github Actions.
  • It is executed everyday to create pull requests to update schema if it the schema changes. One for breaking change and one for backward compatible change. The maintainer can review the PRs to merge one of them. For example: Update schema Atry/hhast#3 and Update schema to a backward incompatible version Atry/hhast#4
  • It is also executed on pull request and will add suggestions to the pull request if it detect the schema needs regeneration.
• src/__Private/codegen/CodegenLastestBreakingVersion.hack is a code generator to generate codegen-no-rebuild/latest_breaking_version.hack, which includes the version number of latest breaking change on AST.
• src/__Private/is_compatible_schema_version.hack is updated to use codegen-no-rebuild/latest_breaking_version.hack to determine whether a schema version is compatible, so we don't have to manually edit the file any more until the 100th century.
• The job determine-latest-breaking-hhvm-version is added to Github Actions to calculate the oldest supported HHVM version so we don't have to manually update .github/workflows/build-and-test.yml any more.
• The job update-schema is also executed when pushing a commit to any branch in either this repository or a forked repository. When a commit is pushed and the schema change is detected, the pull requests in that repository are created against that branch. For example, 4d3644c and 4bb47c4 in this PR are created by dogfooding PRs from Update schema to a backward incompatible version Atry/hhast#4 .