syntax: stop embedding Pos in error struct types

Otherwise the error types gain methods like String or Offset,
which don't make any sense on an error as a whole.
Particularly String, as it won't stringify the entire error;
just its position, omitting the rest of the fields, unlike Error.

This is technically a breaking change, but I never intended the methods
to be included in the error types, and hopefully noone relies on that.
Anyone who relied on them likely got confused by my API mistake here,
and might have buggy code as a result of it.

syntax/parser.go

@@ -803,8 +803,8 @@ func IsKeyword(word string) bool {
 // the parser cannot recover.
 type ParseError struct {
 	Filename string
-	Pos
-	Text string
+	Pos      Pos
+	Text     string
 
 	Incomplete bool
 }
@@ -821,9 +821,9 @@ func (e ParseError) Error() string {
 // in the current language variant, and what languages support it.
 type LangError struct {
 	Filename string
-	Pos
-	Feature string
-	Langs   []LangVariant
+	Pos      Pos
+	Feature  string
+	Langs    []LangVariant
 }
 
 func (e LangError) Error() string {