Add support for inline maps in SpEL expressions

This commit introduces the ability to specify an inline map in
an expression. The syntax is similar to inline lists and of
the form: "{key:value,key2:value}". The keys can optionally
be quoted. The documentation is also updated with information
on the syntax.

Issue: SPR-9472

src/asciidoc/index.adoc

@@ -11058,6 +11058,7 @@ The expression language supports the following functionality
 * Bean references
 * Array construction
 * Inline lists
+* Inline maps
 * Ternary operator
 * Variables
 * User defined functions
@@ -11562,7 +11563,22 @@ Lists can be expressed directly in an expression using `{}` notation.
 entirely composed of fixed literals then a constant list is created to represent the
 expression, rather than building a new list on each evaluation.
 
+[[expressions-inline-maps]]
+==== Inline Maps
+Maps can also be expressed directly in an expression using `{key:value}` notation.
 
+[source,java,indent=0]
+[subs="verbatim,quotes"]
+----
+	// evaluates to a Java map containing the two entries
+	Map inventorInfo = (Map) parser.parseExpression("{name:'Nikola',dob:'10-July-1856'}").getValue(context);
+
+	Map mapOfMaps = (Map) parser.parseExpression("{name:{first:'Nikola',last:'Tesla'},dob:{day:10,month:'July',year:1856}}").getValue(context);
+----
+`{:}` by itself means an empty map. For performance reasons, if the map is itself composed
+of fixed literals or other nested constant structures (lists or maps) then a constant map is created
+to represent the expression, rather than building a new map on each evaluation. Quoting of the map keys
+is optional, the examples above are not using quoted keys.
 
 [[expressions-array-construction]]
 ==== Array construction