Support UTF-16 word lists

[Jason3S]

I had this exact problem when I tried to do the same thing with the Windows system spellcheck-dictionary, with another wrinkle added as well: Code Spell Checker does not like encodings other than UTF-8, and tried to interpret the UTF-16 .dic file (and insert text into it!) as UTF-8.

Originally posted by @DizzyHSlightlyVoided in https://github.com/streetsidesoftware/vscode-spell-checker/issues/1930#issuecomment-1120247801