feat: add ImportExpression node, better esm support.

[morganney]

If this gets merged, you should publish under a new version, ideally a new major version since the package now expects a particular node runtime, i.e. "engines": { "node": ">=12.17.0" } which may be a breaking change for some consumers (but I think it is unlikely).

• Adds "type": "module" to the package.json making it an ESM package, and updates file extensions in import statements.
• Updates babel.config.js and associated build in prepare script to better support ESM and CJS consumers.
• Removes unused dependencies.
• Fixes SyntaxError: Cannot use import statement outside a module #38.
• Fixes TypeError: traveler[child.type] is not a function #39.
• Closes require('astravel') broken due to @babel/runtime dependency #34.