use async read of JSON files on startup

[capaj]

currently https://github.com/ChristianKohler/PathIntellisense/blob/2ff1ac9d981706ac17fb24348b8331e32204406d/src/utils/config.ts#L31-L35

uses readFileSync which blocks all JS execution until all JSON files are read. This is not good for performance, this should be changed to async fs read.