bpo-40939: Rename PyPegen* functions to PyParser* (GH-21016)

Rename PyPegen* functions to PyParser*, so that we can remove the
old set of PyParser* functions that were using the old parser.

diff --git a/Include/parser_interface.h b/Include/parser_interface.h
new file mode 100644
index 0000000..1c6576d
--- /dev/null
+++ b/Include/parser_interface.h
@@ -0,0 +1,52 @@
+#ifndef Py_PEGENINTERFACE
+#define Py_PEGENINTERFACE
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "Python.h"
+
+#ifndef Py_LIMITED_API
+PyAPI_FUNC(struct _mod *) PyParser_ASTFromString(
+    const char *str,
+    const char *filename,
+    int mode,
+    PyCompilerFlags *flags,
+    PyArena *arena);
+PyAPI_FUNC(struct _mod *) PyParser_ASTFromStringObject(
+    const char *str,
+    PyObject* filename,
+    int mode,
+    PyCompilerFlags *flags,
+    PyArena *arena);
+PyAPI_FUNC(struct _mod *) PyParser_ASTFromFile(
+    FILE *fp,
+    const char *filename,
+    const char* enc,
+    int mode,
+    const char *ps1,
+    const char *ps2,
+    PyCompilerFlags *flags,
+    int *errcode,
+    PyArena *arena);
+PyAPI_FUNC(struct _mod *) PyParser_ASTFromFileObject(
+    FILE *fp,
+    PyObject *filename_ob,
+    const char *enc,
+    int mode,
+    const char *ps1,
+    const char *ps2,
+    PyCompilerFlags *flags,
+    int *errcode,
+    PyArena *arena);
+PyAPI_FUNC(struct _mod *) PyParser_ASTFromFilename(
+    const char *filename,
+    int mode,
+    PyCompilerFlags *flags,
+    PyArena *arena);
+#endif /* !Py_LIMITED_API */
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_PEGENINTERFACE */