osk-layouts: Add additional keys to Thai layout

Add Anghankhu (๚), Fongman (๏), Khomut (๛), and Yamakkan ( ๎) keys.

These keys do not exist in most, if not all, Thai physical keyboards,
but may be used by those who study ancient Thai texts. However,
they are an optional part of the TIS-820-2538 spec[0].

[0] https://www.nectec.or.th/it-standards/std820/std820.html

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3106

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1427>

data/osk-layouts/th.json

@@ -176,7 +176,11 @@
             "-",
             ":",
             "'",
-            "@"
+            "@",
+            " ๎",
+            "๏",
+            "๛",
+            "๚"
           ]
         ]
       ]
@@ -347,7 +351,11 @@
             "-",
             ":",
             "'",
-            "@"
+            "@",
+            " ๎",
+            "๏",
+            "๛",
+            "๚"
           ]
         ]
       ]