Add a helper function to inspect MyST AST objects in JavaScript (e.g. in utils)

[choldgraf]

It's useful to be able to print log statements about the contents of AST, JavaScript objects, etc during a MyST build.
However, for print AST objects, using console.log isn't that useful.

If you do

console.log(ast)

It only generates the top layer of AST so you can't see what's in the children etc.

There are two other ways to print the full AST, but both are a pain to remember:

console.dir(ast, { depth: null, colors: true });

and

console.log(JSON.stringify(ast, null, 2))

Suggestion: make this a utility function

We should provide developers a utility function inside of utils or ctx that shows the contents of AST in a user-friendly way.

For example:

utils.inspect(ast, depth=3)