Add FS-1337 - Ignore file-based-app directives

RFCs/FS-1337-ignore-file-based-app-directives.md

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+# F# RFC FS-1337 - Ignore file-based-app directives
+
+The design suggestion [Ignore file-based-app directives](https://github.com/fsharp/fslang-suggestions/issues/1440) has been [marked](https://github.com/fsharp/fslang-suggestions/issues/1440#issuecomment-3932712300) "approved in principle".
+
+This RFC covers the detailed proposal for this suggestion.
+
+- [x] [Suggestion](https://github.com/fsharp/fslang-suggestions/issues/1440)
+- [x] Approved in principle
+- [ ] Implementation
+- [ ] Discussion
+
+# Summary
+
+This RFC specifies that source lines starting with `#:` will be ignored by the compiler.
+
+# Motivation
+
+As of SDK 10.0, the `dotnet run` command supports "file based apps". Currently, only C# is supported. A prerequisite for F# support is that the F# compiler ignores the necessary directives for such apps. If file-based apps are ever introduced for F#, they should use the same syntax as C# does, namely source code lines that start with `#:`. Therefore, this RFC specifies that source lines starting with `#:` will (for now) be ignored by the compiler. This will enable experimenting with F# file-based apps.
+
+This RFC does not specify syntax or semantics of potential future directives starting with that prefix.
+
+
+# Detailed design
+
+Source lines starting with `#ident` for any identifier `ident` beyond the known F# hash directive identifiers (like `if`, `line`, `nowarn`, `r` etc) are ignored already by the compiler.
+This RFC specifies that also directives starting with `#:` are ignored.
+
+
+```fsharp
+#: whatever follows in this line is ignored
+```
+
+# Changes to the F# spec
+
+An additional entry should be added to section [12.4](https://fsharp.github.io/fslang-spec/program-structure-and-execution/#124-compiler-directives) of the F# spec.
+
+# Drawbacks
+
+None
+
+# Alternatives
+
+Hash-ident directives like `#r_project` could be used. They are already ignored today. However, this would mean an unnecessary deviation from the way file-based-apps work in C#.
+
+# Prior art
+
+C# file-based apps.
+
+# Compatibility
+
+None.
+
+# Interop
+
+None
+
+# Pragmatics
+
+## Diagnostics
+
+This RFC is only about ignoring the directives. Diagnostics would possibly be provided by actual use of them.
+
+## Tooling
+
+None so far.
+
+## Performance
+
+None.
+
+## Scaling
+
+No issue.
+
+## Culture-aware formatting/parsing
+
+No issues.
+
+# Unresolved questions
+
+None.