schemagen: add C++20 keywords to the C++ keyword allow-list

Add concept, consteval, constinit, co_await, co_return, co_yield,
requires, module, and import to _CPP_KEYWORDS so property names that
happen to be C++20 keywords get sanitized with a trailing underscore.

Closes #18

contrib/schemagen/weaseljson_schemagen.py

@@ -133,9 +133,15 @@ _CPP_KEYWORDS = {
     "catch",
     "char",
     "class",
+    "concept",
     "const",
+    "consteval",
+    "constinit",
     "constexpr",
     "continue",
+    "co_await",
+    "co_return",
+    "co_yield",
     "decltype",
     "default",
     "delete",
@@ -152,9 +158,11 @@ _CPP_KEYWORDS = {
     "friend",
     "goto",
     "if",
+    "import",
     "inline",
     "int",
     "long",
+    "module",
     "namespace",
     "new",
     "not",
@@ -166,6 +174,7 @@ _CPP_KEYWORDS = {
     "public",
     "register",
     "return",
+    "requires",
     "short",
     "signed",
     "sizeof",