Add .C for valid C++ extensions (#222)

test plan:
see related semgrep PR


- [x] I ran `make setup && make` to update the generated code after
editing a `.atd` file (TODO: have a CI check)
- [x] I made sure we're still backward compatible with old versions of
the CLI.
For example, the Semgrep backend need to still be able to *consume* data
generated
	  by Semgrep 1.17.0.
See
https://atd.readthedocs.io/en/latest/atdgen-tutorial.html#smooth-protocol-upgrades

Author: aryx

Language.ml

@@ -144,7 +144,7 @@ let list = [
   id_string = "cpp";
   name = "C++";
   keys = [{|cpp|}; {|c++|}];
-  exts = [{|.cc|}; {|.cpp|}; {|.cxx|}; {|.c++|}; {|.pcc|}; {|.tpp|}; {|.h|}; {|.hh|}; {|.hpp|}; {|.hxx|}; {|.inl|}; {|.ipp|}];
+  exts = [{|.cc|}; {|.cpp|}; {|.cxx|}; {|.c++|}; {|.pcc|}; {|.tpp|}; {|.C|}; {|.h|}; {|.hh|}; {|.hpp|}; {|.hxx|}; {|.inl|}; {|.ipp|}];
   maturity = Alpha;
   example_ext = Some {|.cpp|};
   excluded_exts = [];

generate.py

@@ -248,7 +248,8 @@ def to_json(self):
         keys=["cpp", "c++"],
         # Extensions are taken directly from tokei's language JSON file:
         #   https://github.com/XAMPPRocky/tokei/blob/master/languages.json
-        exts=[".cc", ".cpp", ".cxx", ".c++", ".pcc", ".tpp", ".h", ".hh", ".hpp", ".hxx", ".inl", ".ipp"],
+        # The .C was added for LSEG
+        exts=[".cc", ".cpp", ".cxx", ".c++", ".pcc", ".tpp", ".C", ".h", ".hh", ".hpp", ".hxx", ".inl", ".ipp"],
         example_ext=".cpp",
         maturity=Maturity.ALPHA,
         shebangs=[]

lang.json

@@ -110,6 +110,7 @@
       ".c++",
       ".pcc",
       ".tpp",
+      ".C",
       ".h",
       ".hh",
       ".hpp",