Support a .flatignore file

[blairdrummond]

It would be handy to extend this:

flat-viewer/src/api/index.ts

Lines 48 to 49 in 19477a0

	const ignoredFiles = ["package.json", "tsconfig.json"];
	const ignoredFolders = [".vscode", ".github"];

[RandomFractals]

.flatignore could be a good solution for #27 and allow github repository maintainers to filter out some json files that flat viewer can't display.

[avanlinden]

I love Flat Viewer and came here to check if there was a way to ignore certain files, since I'd love to give collaborators a flat link to my repos and not have them land by default on a devcontainer or other json file that can't be displayed. Just chiming in that I would also greatly appreciate this feature!