Fix ESP32-S31-WROOM-3 symbol failing to load in KiCad

The easyeda2kicad.py-generated file had an unquoted URL as the
(generator ...) value: '(generator https://github.com/uPesy/...)'.
Bare/unquoted s-expression tokens can't contain ':' or '/', which broke
KiCad's parser and made the whole library silently fail to load (the
symbol was present on disk and correctly registered in fp/sym-lib-table,
but invisible in KiCad). Wrapped the value in quotes.

Also added a check_library.py rule to catch this class of error (an
unquoted generator value containing ':' or '/') automatically going
forward.

symbols/microcontrollers/ESP32-S31-WROOM-3.kicad_sym

@@ -1,6 +1,6 @@
 (kicad_symbol_lib
   (version 20211014)
-  (generator https://github.com/uPesy/easyeda2kicad.py)
+  (generator "https://github.com/uPesy/easyeda2kicad.py")
 
   (symbol "ESP32-S31-WROOM-3_C9900281993"
     (in_bom yes)