Update fcs to 43.8.200-preview.23516.2

[dawedawe]

WHAT

🤖 Generated by Copilot at 7ea80d4

Updated FCS dependency to test F# 6.0 features and added a new method to RoslynSourceTextFile to extract code snippets from a file based on a range.

🤖 Generated by Copilot at 7ea80d4

GetSubTextFromRange Extracts snippets from files Useful for edits

📝🆕🌙

WHY

As with #1192 the tests time out/get stuck at some point. Need to dig into that.

The tests

HOW

🤖 Generated by Copilot at 7ea80d4

• Enable FCS nightly feed source and use newer preview version of FCS package to test new F# 6.0 features and fixes (link, link)
• Add GetSubTextFromRange method to RoslynSourceTextFile type to extract code snippets from a file based on a range of lines and columns (link)