Validate existence + vanilla file-ness of chosen or typed-in SSL PEM or JKS path

[jlrobins]

When SSL PEM or JKS file path is typed it, need to have extension-side validate that the path is stat-able, the user has read perms on it, and that it stats to a vanilla file and not a directory or a socket path.

Basically, call https://nodejs.org/en/learn/manipulating-files/nodejs-file-stats and ensure stats.isFile().

If any fails, then can throw up an immediate polite error message around the file path input.