Declare supported export formats in backend

[simoncozens]

Fixes #1878. FontraPak PR incoming.

Also makes "find-glyphs-that-use-glyph" conditional on the server supporting it.

[justvanrossum]

I'm not sure I like the addition of the supportedExportFormats field to the protocol while exportAs() is entirely optional. Perhaps it's better to have an additional optional partner method called getSupportedExportFormats().

Ideally the changes should not affect https://github.com/googlefonts/fontra-rcjk/blob/main/src/fontra_rcjk/projectmanager.py

[simoncozens]

Fair enough, yes. Will rethink.

[justvanrossum]

The EXPORT_FORMATS constant should probably be deleted, and this snippet can also use the dynamic list of export types:

fontra/src/fontra/views/editor/editor.js

Lines 737 to 748 in 0c9659c

	getItems: () => {
	if (this.fontController.backendInfo.projectManagerFeatures["export-as"]) {
	return [
	{
	title: translate("menubar.file.export-as"),
	getItems: () =>
	EXPORT_FORMATS.map((format) => ({
	actionIdentifier: `action.export-as.${format}`,
	})),
	},
	];
	} else {