Offsets should be UTF-16 code units

[puremourning]

Describe the bug

The DAP uses utf-16 code units for offsets (ref: microsoft/debug-adapter-protocol#91).

Vimspector uses probably a mix of byte offsets and unicode codepoints.

To Reproduce

Add a line with a multi-byte character and a language (like TCL) that provides the column offset.

Expected behavior

Offsets are interpreted correctly according to the appropriate notion of "characters"

Actual behaviour

Not sure yet, i haven't tried it, but i expect tracebacks

[puremourning]

This has been clarified in microsoft/debug-adapter-protocol#285; the result is the same, it's UTF16 code units. Shrug.