…owerShell#12658)

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# PR Summary

Typed return value for SessionStateInternal.GetFunctionTable.
IDictionary<string, FunctionInfo> instead of IDictionary.

## PR Context

Removes unnecessary casts, and helps with later nullablility changes.

## PR Checklist

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- **[Breaking changes](https://github.com/PowerShell/PowerShell/blob/master/.github/CONTRIBUTING.md#making-breaking-changes)**
    - [x] None
    - **OR**
    - [ ] [Experimental feature(s) needed](https://github.com/MicrosoftDocs/PowerShell-Docs/blob/staging/reference/6/Microsoft.PowerShell.Core/About/about_Experimental_Features.md)
        - [ ] Experimental feature name(s): <!-- Experimental feature name(s) here -->
- **User-facing changes**
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    - [x] [Make sure you've added a new test if existing tests do not effectively test the code changed](https://github.com/PowerShell/PowerShell/blob/master/.github/CONTRIBUTING.md#before-submitting)
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        - Impact on [EditorSyntax](https://github.com/PowerShell/EditorSyntax) (which provides syntax highlighting with in VSCode, GitHub, and many other editors).