Split checker into multiple files

[mraak]

For the purposes of better readability, easier branching merging and easier developer onboarding, it would be useful to split the checker.js into multiple files.

For example,

var collectTypesVisitors = {
	TaggedTemplateExpression(path) {
		if (T.isIdentifier(path.node.tag)) {
			let tagName = path.node.tag.name;
			if (recognizedTypeIDs.includes(tagName)) {
				nodeTypes.set(path.node,{ tagged: tagName, });
			}
		}
	},
	TemplateLiteral(path) {
		if (T.isTaggedTemplateExpression(path.parent)) {
			let parentType = nodeTypes.get(path.parent);
			if (parentType) {
				nodeTypes.set(path.node,{ ...parentType, });
			}
			else {
				nodeTypes.set(path.node,{ inferred: "unknown", });
			}
		}
		else {
			nodeTypes.set(path.node,{ inferred: "string", });
		}
	},

//... etc...
}

could be split on a "per visitor type" basis.

var collectTypesVisitors = {
  TaggedTemplateExpression: require('checkTaggedTemplateExpression'),
  TemplateLiteral: require('checkTemplateLiteral'),
}

Or something similar.

That way for example, a developer or a group of developers could focus on documentation, tests, and code only related to one visitor type, without ever affecting the rest of the code.