Cleanup related to diagnostics, ParsedInput

[dsyme]

This extracts out some cleanup from #11057

• Remove NumberedError, it is the same as Error

• Use FSharpDiagnosticSeverity everywhere instead of a boolean isError flag

• Move some things into CompilerImports.fs from fsc.fs which gives better layering

• Make parsing always return a ParsedInput rather than a ParsedInput option, creating an empty ParsedInput in the (very rare) cases where parsing outright failed. This makes subsequent processing much simpler

• Move FSharpProjectOptions into FSharpCheckerResults.fs/fsi. THis is a better location as it means the FSharpCheckerResults can report the options as part of the project context

[dsyme]

Replaced by #11151