libsyntax: Forbid escapes in the inclusive range \x80-\xff in

Unicode characters and strings.

Use `\u0080`-`\u00ff` instead. ASCII/byte literals are unaffected.

This PR introduces a new function, `escape_default`, into the ASCII
module. This was necessary for the pretty printer to continue to
function.

RFC #326.

Closes #18062.

[breaking-change]

src/etc/unicode.py

@@ -283,7 +283,7 @@ def load_east_asian_width(want_widths, except_cats):
     return widths
 
 def escape_char(c):
-    if c <= 0xff:
+    if c <= 0x7f:
         return "'\\x%2.2x'" % c
     if c <= 0xffff:
         return "'\\u%4.4x'" % c