ConvertOffsetToPoint, ConvertLineAndColumnToOffset do not utilize LengthInBufferCells for ContinuationPrompt

[msftrncs]

In the source for ConvertOffsetToPoint(), referencing the length of ContinuationPrompt doesn't account for its LengthInBufferCells.

PSReadLine/PSReadLine/Render.cs

Line 1038 in 6f0ccf4

var continuationPromptLength = Options.ContinuationPrompt.Length;

[msftrncs]

Include ConvertLineAndColumnToOffset() in this as well.

PSReadLine/PSReadLine/Render.cs

Line 1098 in 6f0ccf4

var continuationPromptLength = Options.ContinuationPrompt.Length;

[msftrncs]

This might be the cause for #1188, as LengthInBufferCells(string) seems to have support for VT color sequences.

/cc: @JustinGrote