py/modsys: Add optional mutable attributes sys.ps1/ps2 and use them.

This allows customising the REPL prompt strings.

Signed-off-by: Damien George <damien@micropython.org>

py/mpstate.h

@@ -41,6 +41,10 @@
 // variable, but in the future it is hoped that the state can become local.
 
 enum {
+    #if MICROPY_PY_SYS_PS1_PS2
+    MP_SYS_MUTABLE_PS1,
+    MP_SYS_MUTABLE_PS2,
+    #endif
     #if MICROPY_PY_SYS_TRACEBACKLIMIT
     MP_SYS_MUTABLE_TRACEBACKLIMIT,
     #endif