Move environment-visitor helper into @babel/traverse

[nicolo-ribaudo]

Q	A
Fixed Issues?	Fixes #1, Fixes #2
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

I'm working on removing cycles between packages caused by type-only imports. My motivation is that, even if it's a type-only import, it's an actual conceptual cycle and those packages will always be used together anyway. For example:

• @babel/traverse depends on @babel/helper-environment-visitor
• @babel/helper-environment-visitor does not explicitly depend @babel/traverse, but it can only be used on NodePaths so it can only be used when @babel/traverse

This has two benefits:

• when publishing TS types we'll need to mark them as actual dependencies, and having cycles between packages causes problems
• we reduce the number of packages

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/57182

[JLHwung]

I recall previously we imported the traverse from their peer dependent @babel/core because of compatibility issues. Specifically we want the core, even if it is an old version, to explode any AST alias so that the traverser, imported from core, will not complain about any new AST types beyond its knowledge.

Now that visitors.environmentVisitor will explode the visitors via merge from the latest traverse, maybe we have to defer this change to Babel 8?