Python: Improve handling of syntax errors

Rather than relying on matching arbitrary nodes inside tree-sitter-graph
and then checking whether they are of type ERROR or MISSING (which seems
to have stopped working in later versions of tree-sitter), we now
explicitly go through the tree-sitter tree, locating all of the error
and missing nodes along the way. We then add these on to the graph
output in the same format as was previously produced by
tree-sitter-graph.

Note that it's very likely that some of the syntax errors will move
around a bit as a consequence of this change. In general, we don't
expect syntax errors to have stable locations, as small changes in the
grammar can cause an error to appear in a different position, even if
the underlying (erroneous) code has not changed.

python/extractor/tsg-python/python.tsg

@@ -6,16 +6,6 @@
 (module) @mod
 { let @mod.node = (ast-node @mod "Module") }
 
-(_) @anynode
-{
-    scan (node-type @anynode) {
-        "^(ERROR|MISSING)$" {
-            let @anynode.node = (ast-node @anynode "SyntaxErrorNode")
-            attr (@anynode.node) source = (source-text @anynode)
-        }
-    }
-}
-
 (parenthesized_expression) @nd
 { let @nd.node = (ast-node @nd "Expr") }