add JS AST querying with XPath

[chalu]

I added a transformation that allows the user query the AST from Javascript, using XPath. Hopefully, this can be the beginning of getting solid AST querying and across board in the AST Explorer tool.

Let me know what you think.
Thanks

[rajasegar]

@chalu
Thanks a lot for this PR, i think it greatly simplifies the AST querying , just had a look at astq repo, it is
super cool. However, we need a little bit info regarding the same like how you can query, etc., Can you please add some context info in README
@fkling I think that is OK if the PR gets merged
Also can you please remove all the .DS_Store files?

[chalu]

Thanks. Sorry about the .DS_Store files.
By README, are you referring to the main README here

I also got curious about a way to provide support for AST querying for other languages, but I guess that depends on first finding a good XPath query library for them.

[rajasegar]

Yes

[chalu]

Kindly review and let me know how the latest changes address the previous concerns and if there's any more feedback.

Thanks.

[brettz9]

This looks very useful--maybe an esquery tool could be added as well (the query tool behind eslint's selectors)!