Add UTF-16 option to Base64 encoder/decoder

[dchristensen]

Adds the UTF-16 encoding type to the Base64 encoder/decoder

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• Bugfix
• New feature or enhancement
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What is the current behavior?

The Base64 encoder/decoder does not support UTF-16 encoding. This encoding is commonly used when working with PowerShell as it supports encoding commands using the powershel.exe -EncodedCommand option.

Issue Number: DevToys-app/DevToys#661

What is the new behavior?

• Adds UTF-16 support

Other information

This only adds support for UTF-16LE with BOM (System.Text.Encoding.Unicode). I considered adding UTF-16LE and UTF-16BE explicitly as well, but felt that might clutter the UI with too many options. I think UTF-16LE BOM will be sufficient for most use-cases (PowerShell supports UTF-16LE with BOM).

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[dchristensen]

I realized I failed to actually output the BOM when encoding a Unicode string. As I've been going through to add that I've found some existing code in the codebase that I have questions about.

In the Base64Helper.FromBase64ToText it looks like we're encoding the UTF-8 preamble (using Encoding.Unicode) and including it in encoded output?
https://github.com/DevToys-app/DevToys.Tools/blob/main/src/DevToys.Tools/Helpers/Base64Helper.cs#L123-L143

I've found the related issue (DevToys-app/DevToys#342) and PR (DevToys-app/DevToys#378) that appears to have driven this change.

I find it strange that the preamble is included in the plain-text output. The Preamble seems like it should be an implementation detail of the encoding when converting characters to bytes. The same string could be encoded with or without a preamble depending on the encoder settings.