Format interpolated expressions as single line (#1247)

This forces iterpolated expressions to be single-line, so that newline
literals within the bounds of two string delimiters can be seen as
verbatime newlines in the resulting string.

Edge case: in the case of a line comment, it's not possible to keep
this as a single line expression.
These are kept as multi-line expressions.

Also:

* Remove `ForceWrap`, this node is not used.
* Rename `StringConstant` -> `StringChars`

pkl-formatter/src/test/files/FormatterSnippetTests/output/multi-line-strings.pkl

@@ -1,8 +1,6 @@
 foo =
   """
-  asd \(new {
-    bar = 1
-  }) asd
+  asd \(new { bar = 1 }) asd
   """
 
 bar =