Nullness feature request - enable nullness checking in .fsx scripts

[T-Gro]

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.
• 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

[T-Gro]

Will need to change how nullness info is inside TcGlobals (if it should support switching on/off)

[T-Gro]

Another solution: Support enabling/disabling nullness warning for blocks of code, the nullness feature would be always on.

[T-Gro]

Resonable default: Enable nullness by default in the IDE, but ignore the warnings (FSI adding some default warnings to be ignored) when running the script.

#warnon would put them back on.

• editor setting (global, not per file) to enable/disable the nullness in the IDE.