Improve the typing of the extension-load-queue

[makhnatkin]

The src/lib/extension-load-queue.ts describes the mechanism for asynchronous loading of executable scripts using packages. This process is achieved through a queue and by adding a field to the window object. Symbol data types are used to protect against external interference. This mechanism is employed in such extensions as mermaid, LaTeX, and HTML.

The file is described using TypeScript. At the same time, it contains a number of areas that could be improved.

Suggestion for improvement

1. fix TODO
2. add docs
3. move the isBrowser function