add symbol hover support for codesearch.debian.net

When we are showing symbol info for a macro or a function in a c file, then add a link to search the source code of that macro/function in codesearch.debian.net .

It uses the codesearch.debian.net regex search with the following regex which looks up source definitions for C macros and functions:

(?mi:^(?:#define\s+)?func_name)\s*((?m:([^;]+)$)

Note: This was implemented in the Symbol hover info because the plugin hooks for extending context menu doesn't let you add/filter an action that is only shown for macro and function symbols in c files . While the Symbol hover card lets you add info for only that case, as this MR does.