FCS: add FSharpCheckFileResults.FileSignature

[auduchinok]

Fixes #3259. Adds access to the file own signature, including memers hidden fsi file.

[github-actions]

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Change path	Release notes path	Description
`src/Compiler`	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md	No current pull request URL (#20478) found, please consider adding it

[T-Gro]

🤖 🕵️ Background FileSignature drops declarations hidden by .fsi with the legacy checker, even though its symbol uses contain those definitions. This needs the fully checked implementation signature rather than the skipped-file placeholder.

// Test.fsi
module Test
val visible: int

// Test.fs
module Test
let hidden = 41
let visible = hidden + 1

// FCS client, with project options for these files:
let checker =
    FSharpChecker.Create(useTransparentCompiler = false,
                         keepAllBackgroundResolutions = true)
let _, result =
    checker.GetBackgroundCheckResultsForFileInProject(testFs, options)
    |> Async.RunSynchronously
let test = result.FileSignature.FindEntityByPath ["Test"] |> Option.get
test.MembersFunctionsAndValues |> Seq.map _.DisplayName |> Seq.toList
// ["visible"]; foreground and transparent checking return ["hidden"; "visible"].

[T-Gro]

🤖 🕵️ Hidden nested types lose their declaring module: DeclaringEntity returns None in both checker modes although the parent is present in FileSignature. The symbol-resolution context needs the inferred contents too; the exported CCU signature cannot resolve that parent.

// Test.fsi
module Test
val visible: int

// Test.fs
module Test
let visible = 1
module Hidden =
    type Secret<'T> = { Item: 'T }

// FCS client, after checking Test.fs:
let hidden =
    result.FileSignature.FindEntityByPath ["Test"; "Hidden"]
    |> Option.get
hidden.NestedEntities[0].DeclaringEntity
// None; without Test.fsi this is Some Test.Hidden.