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/unification/solvers/unification_test.go

@@ -859,6 +859,11 @@ func TestUnification1(t *testing.T) {
 
 			data := &interfaces.Data{
 				// TODO: add missing fields here if/when needed
+				Metadata: &interfaces.Metadata{},
+				SourceFinder: func(string) ([]byte, error) {
+					return nil, fmt.Errorf("not implemented")
+				},
+
 				Debug: testing.Verbose(), // set via the -test.v flag to `go test`
 				Logf: func(format string, v ...interface{}) {
 					t.Logf(fmt.Sprintf("test #%d", index)+": ast: "+format, v...)