patch 8.2.4883: string interpolation only works in heredoc

Problem:    String interpolation only works in heredoc.
Solution:   Support interpolated strings.  Use syntax for heredoc consistent
            with strings, similar to C#. (closes #10327)
diff --git a/src/eval.c b/src/eval.c
index 0c6d2e5..18e196f 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -3769,8 +3769,12 @@
 
     /*
      * Environment variable: $VAR.
+     * Interpolated string: $"string" or $'string'.
      */
-    case '$':	ret = eval_env_var(arg, rettv, evaluate);
+    case '$':	if ((*arg)[1] == '"' || (*arg)[1] == '\'')
+		    ret = eval_interp_string(arg, rettv, evaluate);
+		else
+		    ret = eval_env_var(arg, rettv, evaluate);
 		break;
 
     /*