Annotate low-level scripting actions in various stores with JSDoc

[veggiesaurus]

Based on request in CARTAvis/carta#78 from @keflavich.

We need to have some form of documentation for the low-level scripting actions that are available. This will allow users to either utilise the debug execution feature on the frontend, or extend the high-level Python scripting interface.

I suggest we use JSDoc for this, as we can embed the documentation in the code and generate an API document in HTML reasonably easily.

As an example, the angus/jsdoc branch has the following for appendFile:

/**     
* @access public
* @async
* Appends a file at the given path to the list of existing open files
* @param path - path to the parent directory of the file to open, or of the file itself     
* @param {string=} filename - filename of the file to open     
* @param {string=} hdu - HDU to open. If left blank, the first image HDU will be opened     
* @return {Promise<number>} [async] the file ID of the opened file     
* */
* 
@action appendFile = (path: string, filename: string, hdu: string) => {
...

If we can document these functions well, and also extend the debug execution dialog into something more akin to a full scripting console, CARTA would become very extensible.