Better intellisense inside of QLDoc comments

[aeisenberg]

In addition to providing code templates for query metadata written in QLDoc blocks (see #392), we should also provide proposals inside of QLDoc comment blocks.

For now, I think all we need to do is propose relevant @ tags in file-level query metadata comments (eg- @kind).

[adityasharad]

Updated terminology to refer to query metadata and QLDoc (as opposed to qhelp which are separate XML files).