Document verbatim interpolated strings using $@ and @$ syntax in F#

[Copilot]

Closes #[issue number]

Summary

This PR documents the ability to combine the $ (interpolation) and @ (verbatim string) prefixes in F# interpolated strings, answering a user question about whether the following syntax is valid:

printfn $@"Migrate notes of file ""{oldId}"" to new file ""{newId}""."

Answer: Yes! F# has supported verbatim interpolated strings using both $@ and @$ prefixes since F# 5, but this feature was not documented.

Changes

Updated docs/fsharp/language-reference/interpolated-strings.md

1. Syntax section - Added two new syntax forms:

   • $@"string-text {expr}"
   • @$"string-text {expr}"

2. Verbatim interpolated strings section - Restructured and expanded to document both approaches:

   • New subsection: "Using $@ or @$ prefix" - Documents how to combine interpolation with verbatim strings, with practical examples showing:
     • File paths with backslashes (no escaping needed)
     • Embedding quotes using ""
     • Multi-line strings
     • Both $@ and @$ order variations
   • Existing content: "Using triple quotes" - Preserved as a subsection for the alternative triple-quote approach

3. Metadata - Updated ms.date to reflect the documentation update

Added docs/fsharp/language-reference/snippets/interpolated-strings/verbatim.fsx

Created a new snippet file with working examples demonstrating verbatim interpolated strings, including multi-line usage and quote embedding.

Testing

All code examples were tested with dotnet fsi and verified to work correctly, including:

• Both $@ and @$ prefix orders
• File paths with backslashes
• Embedded quotes using ""
• Multi-line strings
• Format specifiers with verbatim interpolated strings

This feature works identically to C#'s verbatim interpolated strings, providing developers with a familiar and convenient syntax for working with strings containing backslashes and quotes.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Can I combine leading @ with interpolated strings</issue_title>
<issue_description>Hello,

Is the following possible in F#?

printfn $@"Migrate notes of file ""{oldId}"" to new file ""{newId}""."

The combination of $ and @.

---

Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: 178dbf92-8d00-887f-f55c-44ae4ce0fc26
• Version Independent ID: fa136057-4782-c009-437b-f08bdd267f6a
• Content: Interpolated strings - F#
• Content Source: docs/fsharp/language-reference/interpolated-strings.md
• Product: dotnet-fsharp
• GitHub Login: @cartermp
• Microsoft Alias: phcart</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #23806

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Internal previews

📄 File	🔗 Preview link
docs/fsharp/language-reference/interpolated-strings.md	docs/fsharp/language-reference/interpolated-strings

[BillWagner]

This LGTM and is ready for final review.