API: `Node` is missing `getChildren()`, `getFirstToken()`, `getLastToken()` and related getters

[oMatheusmol]

Description

The native API's Node is missing the child/token getters that exist on the JS-based API: getChildren(), getChildCount(), getChildAt(), getFirstToken() and getLastToken(). These are useful for API consumers (e.g. tools that walk the syntax tree including tokens), and are a natural completion of the position/text getters that were already added.

History

This work started in the typescript-go repo before development moved back here:

• Tracking issue: Node is missing getFullStart(), getStart() and other getters typescript-go#4216 (Node is missing getFullStart(), getStart() and other getters)
• First part (position/text getters) was merged as API: add the missing position and text getters to Node typescript-go#4308
• The remaining getters were implemented in API: add getChildren and token getters to Node typescript-go#4440, which was closed due to the repository move, with a request from the maintainers to re-open the PR here

I'm opening this issue to track the re-submission; the PR follows shortly.