Fix string concatenation spacing in line formatter

[jakebailey]

I thought of this case while reading the fstring spec. The line formatter defaults to not spacing around string literals, but it's likely better to insert a space between two string literals next to each other.

For reference:

• autopep8 leaves spacing as-is, so 'a''b' and 'a' 'b' would not be changed (even with more than one space between). I could emulate this behavior in the line formatter, since TokenExt includes PreceedingWhitespace.
• yapf ensures there is a space between two strings.
• black behaves the same as yapf (barring its replacement of ' with ").

[AlexanderSher]

Does next == null means that it is the last token?
Also, do you need to check if they are on the same line?

[jakebailey]

Yes, that next would be null if the current token were the last token on the line. That next is pulled from the list of tokens of the line, not the linked-list that's happening in TokenExt which could peer forward. I wouldn't want to look at the next line, since even if the next token (ignoring whitespace) happens to be a string literal, the fact that it's on another line implies that there's already spacing and adding a space here would only be adding trailing whitespace (which would then be trimmed away).