Better content assist inside of comments

[aeisenberg]

Inside of a QLDoc, there is structure and the editor can be more helpful at providing better content assist support for comments and tags.

Documentation on QLDoc: https://help.semmle.com/QL/ql-spec/qldoc.html

At a minimum, the editor should propose completion of well known tags, e.g. all the tags here-

/**
 * @name Useless assignment to local variable
 * @description An assignment to a local variable that is not used later on, or whose value is always
 *              overwritten, has no effect.
 * @kind problem
 * @problem.severity warning
 * @id cs/useless-assignment-to-local
 * @tags maintainability
 *       external/cwe/cwe-563
 * @precision very-high
 */

Also, since QLDoc accepts markdown syntax, we should auto-close markdown-ish tags, like backticks and _

This task will probably require changes to the language server.