Add source location to HelperOptions

[karlvr]

The options object passed to a helper contains more information than in the type HelperOptions, specifically name, loc and lookupProperty. It's easy enough to add these to the type definition:

	name: string
	loc: {
		start: hbs.AST.Position
		end: hbs.AST.Position
	}
	lookupProperty: (object: unknown, propertyName: string) => unknown

But the loc property isn't a full SourceLocation. It's missing the source: string property, which I hope will identify the filename of the current template. It would be fantastic to add that property so that a helperMissing helper can produce more useful output!