Add CI: build + parser conformance + ESLint-plugin compatibility

[ericsssan]

There's no GitHub Actions workflow. The README makes strong, checkable claims that currently aren't guarded against regressions:

• test262 parser: 3,966/3,966
• ESLint core: 292/292 rules
• eslint-plugin-n: 43/43, eslint-plugin-es-x: 208/208, eslint-plugin-import: 45/46

Action: add a workflow that builds the Zig native module and runs the parser-conformance + ESLint-plugin compatibility suites on each push/PR, so those numbers stay honest. es-parser, zbc, and zquic already have CI to model this on.