This PR provides completion list for the following cases:

• List all the JsDoc tag names right after the @ sign or in the tagname, e.g.:

/** @^ */
var foo = ...

and

/** @pa^ */
var foo = ...

• Full intellisense in certain JsDoc tag braces, e.g.:

/** @type { numbe^ } */
var foo = ...

/** @param { numbe^ } bar */
var foo = ...

The initial implementation only applies to js files, as currently the parser stores the JSDocComment nodes in the tree for js files but not ts files. Support for ts files may need some discussion.