initial goto impl for signature file

[TheAngryByrd]

WHAT

🤖 Generated by Copilot at cf01557

This pull request improves the support for signature files in FsAutoComplete by adding helper functions to convert file names and filtering out irrelevant symbols from code completion and tooltips. It affects the ProcessHelper and Commands modules in the src/FsAutoComplete.Core folder.

🤖 Generated by Copilot at cf01557

To support signature files in F#
We filter symbols with some zest
We use filterSymbols and ProcessHelper
To match the files with .fsi and .fs
And avoid showing duplicates or incorrect stuff

📝🔎🛠️

WHY

HOW

🤖 Generated by Copilot at cf01557

• Add support for signature files in code completion and tooltips by filtering out irrelevant symbols (link, link, link)

• In src/FsAutoComplete.Core/Commands.fs, modify filterSymbols to check if the file is a signature file (.fsi) and only keep the symbols that match the implementation file (.fs) (link)
• In src/FsAutoComplete.Core/Utils.fs, add helper functions toSignatureFile and toFSharpFile to convert file names between signature and implementation files (link, link)