Port trivia line by line C# -> VB

[GrahamTheCoder]

• A rewrite of comment conversion code which could handle VB -> C# and C# -> VB, but not very reliably, so was never activated for C# -> VB in production.
• This PR will focus on implementing a conversion for C# -> VB - fixes C# -> VB code comments are not converted #8
• A later PR will replace the VB -> C# comment conversion with the same mechanism used here

The focus here is not losing any comments - getting all of them into the result in roughly the order, and within a line or so of where they were in the source. To understand why this is a bit vague, take the example of a for loop in C#, which sometimes needs to turn into a while loop in VB:

Some source code:

for(double y = 0d; y < height; y += 10d) // 10 is the same amount we used earlier
    Draw(y);

for(double y = 0d; y < height; y += 10d) // don't go past height
    Draw(y);

The conversion we might hope for:

While y < height
    Draw(y)
    y += 10R ' 10 is the same amount we used earlier
End While

While y < height ' don't go past height
    Draw(y)
    y += 10R
End While

Obviously we can't know whether the comment was regarding the condition or the increment expression. This PR keeps the comment with the closest expression wherever possible. (i.e. the first case will work, the second will look a bit wrong)

To do

• Handle method body comments
• Source mapping markers for identifiers
• Implement new test helper (add a line comment on every line, assert the output contains all the line comments on their own line and in order)
• Source mapping markers for statements (to get ends of blocks working)
• Source mapping for imports
• Deal with multiple source lines mapping to the same target line
• Tidy up any obvious edge cases, put others out of scope