fix(erlang): address functions by name/arity without the name path separator

Erlang LS identifies functions, types and parameterised macros as
`name/arity`, but `/` separates the components of a Serena name path, so
`create_user/4` was parsed as "symbol `4` nested inside `create_user`" and
could never match -- not even via the name path Serena itself reported for
the symbol. Browsing still worked, but find_referencing_symbols,
replace_symbol_body and insert_after_symbol were unusable on Erlang
functions.

Normalize the name to `create_user#4` instead. `#` cannot occur in an
unquoted Erlang atom, so it can never collide with a real name, unlike `@`.
The arity is kept rather than stripped because it is part of a function's
identity in Erlang: create_order/3 and create_order/2 are different
functions and may coexist in one module.

Also document the "no `/` in symbol names" rule on _normalize_symbol_name,
which is what the next backend for such a language needs to know.

docs/01-about/020_programming-languages.md

@@ -68,7 +68,8 @@ Some languages require additional installations or setup steps, as noted.
 * **Elm**  
   (requires Elm compiler)
 * **Erlang**  
-  (requires installation of beam and [erlang_ls](https://github.com/erlang-ls/erlang_ls); experimental, might be slow or hang)
+  (requires installation of beam and [erlang_ls](https://github.com/erlang-ls/erlang_ls); experimental, might be slow or hang;
+  note that functions are addressed as `name#arity`, e.g. `create_user#4`, because `/` is reserved as the name path separator)
 * **F#**  
   (requires [.NET v8.0+](https://dotnet.microsoft.com/en-us/download/dotnet); uses FsAutoComplete/Ionide, which is auto-installed; for Homebrew .NET on macOS, set DOTNET_ROOT in your environment)
 * **Fortran**