Recognize .inl as a C++ header file extension

.inl is not recognized as C++ file.
This leads to follow up issues, specific in clangd.

clangd analyzes your synthax, so it reads the files clang is given. I assume it filters files ahead, so .hpp and .cpp is recognized as C++ files. But it seems to not do the same with .inl. This leads to several follow up issues, specific clangd being unable to analyse .inl .

**Fix should be assumingly easy, adding .inl to the whitelist.**

Start by tracing clangd's file-extension filtering, comparing how .hpp and .cpp are recognized with the handling of .inl. Locate the whitelist or equivalent entry point and the related tests, then verify that .inl is treated as C++ and that clangd can analyze it.

Written by the indexing model from the issue text.