Add explicit clang-format file with P4C style (#64)

Signed-off-by: Anton Korobeynikov <[email protected]>

Author: asl

.clang-format

@@ -0,0 +1,30 @@
+---
+Language: Cpp
+BasedOnStyle: Google
+IndentWidth: 4
+FixNamespaceComments: true
+ColumnLimit: 100
+SpacesBeforeTrailingComments: 2
+AccessModifierOffset: -3
+
+IncludeBlocks: Regroup
+# Force pointers to the type for C++.
+DerivePointerAlignment: false
+PointerAlignment: Right
+Standard: c++17
+IncludeCategories:
+  # Matches common headers first and sorts them before project includes
+  - Regex: '^<.+/.*\.h>'
+    Priority: 2000
+  - Regex: "^<.+/.*>"
+    Priority: 4000
+  - Regex: '^<.*\.h>'
+    Priority: 1000
+  - Regex: "^<.*>"
+    Priority: 3000
+  - Regex: "testgen/.*"
+    Priority: 6000
+  - Regex: ".*"
+    Priority: 5000
+IncludeIsMainRegex: '(_test)?$'
+...