clang-format: use AngleBracket for main includes

This project uses angle brackets instead of quotes for project-specific
headers. Setting MainIncludeChar enables clang-format to automatically
detect the main header, so it can be kept as the top group of includes.

For example, without this change, the below command would demote
<signet.h> from being the main header. With this change, the order is
preserved.

`clang-format -i src/signet.cpp`

src/.clang-format

@@ -142,6 +142,7 @@ LambdaBodyIndentation: Signature
 LineEnding:      DeriveLF
 MacroBlockBegin: ''
 MacroBlockEnd:   ''
+MainIncludeChar: AngleBracket
 MaxEmptyLinesToKeep: 2
 NamespaceIndentation: None
 ObjCBinPackProtocolList: Auto