lang: ast, gapi, interfaces, parser: Print line numbers on error

This adds an initial implementation of printing line numbers on type
unification errors. It also attempts to print a visual position
indicator for most scenarios.

This patch was started by Felix Frank and finished by James Shubin.

Co-authored-by: Felix Frank <Felix.Frank.de@gmail.com>

lang/lang.go

@@ -189,6 +189,10 @@ func (obj *Lang) Init(ctx context.Context) error {
 		LexParser:       parser.LexParse,
 		Downloader:      nil, // XXX: is this used here?
 		StrInterpolater: interpolate.StrInterpolate,
+		SourceFinder: func(string) ([]byte, error) {
+			// We're running a bundle as part of a deploy.
+			return nil, fmt.Errorf("not implemented") // XXX: read from the fs?
+		},
 		//Local: obj.Local, // TODO: do we need this?
 		//World: obj.World, // TODO: do we need this?