Nullness feature request - enable nullness checking in .fsx scripts

### Issue description

Since .fsx files do not have project options, it is not possible to pass in `/checknulls+` except for passing command line switches directly.
Make it possible (`#nullness-checking` pragma and/or IDE setting) to turn it on for scripting.

### Choose one or more from the following categories of impact

- [ ] Unexpected nullness warning (false positive in nullness checking, code uses --checknulls and langversion:preview).
- [ ] Missing nullness warning in a case which can produce nulls (false negative, code uses --checknulls and langversion:preview).
- [ ] Breaking change related to older `null` constructs in code not using the checknulls switch.
- [ ] Breaking change related to generic code and explicit type constraints (`null`, `not null`).
- [ ] Type inference issue (i.e. code worked without type annotations before, and applying the --checknulls enforces type annotations).
- [ ] C#/F# interop issue related to nullness metadata.
- [X] Other (none of the categories above apply).

### Operating System

Windows (Default)

### What .NET runtime/SDK kind are you seeing the issue on

.NET SDK (.NET Core, .NET 5+)

### .NET Runtime/SDK version

.NET9

### Reproducible code snippet and actual behavior

_No response_

### Possible workarounds

_No response_