feat(developer): ldml - support local imports

[mcdurdin]

only base=cldr is currently supported but spec allows local relative/absolute imports.

keyman/common/web/types/src/ldml-keyboard/ldml-keyboard-xml-reader.ts

Lines 187 to 190 in 5dbda00

	if (base !== constants.cldr_import_base) {
	this.callbacks.reportMessage(CommonTypesMessages.Error_ImportInvalidBase({base, path, subtag}));
	return false;
	}

Reported by @sovichet

[srl295]

• define project-relative imports? kmc-relative?
• command line option to add additional -I include paths?

[srl295]

blocked by #11236