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)

runtime/doc/eval.txt

@@ -1523,6 +1523,25 @@ to be doubled.  These two commands are equivalent: >
 	if a =~ '\s*'
 
 
+interpolated-string					*interp-string* *E256*
+--------------------
+$"string"		interpolated string constant		*expr-$quote*
+$'string'		interpolated literal string constant	*expr-$'*
+
+Interpolated strings are an extension of the |string| and |literal-string|,
+allowing the inclusion of Vim script expressions (see |expr1|).  Any
+expression returning a value can be enclosed between curly braces.  The value
+is converted to a string.  All the text and results of the expressions
+are concatenated to make a new string.
+
+To include an opening brace '{' or closing brace '}' in the string content
+double it.
+
+Examples: >
+	let your_name = input("What's your name? ")
+	echo $"Hello, {your_name}!"
+	echo $"The square root of 9 is {sqrt(9)}"
+
 option						*expr-option* *E112* *E113*
 ------
 &option			option value, local value if possible