GPUCompilationMessage: Specify linePos units, consider span #1597

• The units of linePos should be defined (this may be implicit from how WGSL parses characters, but should be mentioned here too) - i.e. bytes, codepoints, grapheme clusters, etc.
• Should there be a span (linePosStart/linePosEnd or linePos/spanLength) for the error?

For reference, GPUCompilationMessage was added in #646.