Update .gitignore with LSP files

Update .gitignore with LSP files

This does not provide files for LSP support, but ignores them so they
aren't accidentally checked in by developers.

LSP (Language Server Protocol) is a tools that can be used with various
editors to make navigating source code easier. It is more advanced than
`cscope` and supports completion, for example.

A common LSP for C/C++ is `clangd`, and it creates a `.cache` directory
within the project to store data.

The tool `bear` can be used with `make` to assist `clangd` in
determining where the source code is (specifically headers). This is
critical as OpenSSL uses the `<>` form of `#include` rather than the
`""` form. The `<>` form will cause `clangd` to look in e.g.
`/usr/include` for header files, rather than `include/openssl`. The
`bear` tool will create `compile_commands.json` that `clangd` can use
to find include files.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21338)