Feature Request: Add a setting to allow source files to be discerned from header files

There are various scenarios in which cpptools will try to discern if a file is a source file or a header file. For example:

• include graph generation - to determine whether a file should be a source node or header node in the graph.
• TU creation - to determine if a header file is requested, and, if so, determine a source file to create the TU with instead.
• code analysis - When a save operation is performed on a source file, code analysis for that file is canceled, to ensure clang-tidy doesn't keep it open/locked. When a header file is saved, all code analysis operations are canceled, as any one of them might be keeping the header open/locked.
• include auto-complete - to determining if a file should be included in the list, or to determine if the includes cache needs to be cleared/updated.
• Goto-def, to determine if a candidate is a source or header candidate.
• (various other places)

This issue tracks adding a setting to allow the user to customize what file extensions (or wildcard patterns, to match files.associations) are considered headers vs. sources, rather than hard-coding these associations.

Start by tracing the existing source-versus-header decisions used by include graph generation, translation-unit creation, code analysis, include auto-complete, and Go to Definition. Define the setting's extension or wildcard behavior and verify that each listed workflow respects it instead of relying on hard-coded associations.