parser: allow struct definitions inside functions

doc/docs.md

@@ -4067,7 +4067,7 @@ Only one `Option` or `Result` is allowed to be returned from a function. It is
 possible to return multiple values and still signal an error.
 
 ```v
-fn multireturn(v int) !(int, int) {
+fn multi_return(v int) !(int, int) {
 	if v < 0 {
 		return error('must be positive')
 	}
@@ -8065,4 +8065,4 @@ Assignment Operators
 &=   |=   ^=
 >>=  <<=  >>>=
 &&= ||=
-```
+```

vlib/v/parser/parser.v

@@ -1157,6 +1157,10 @@ fn (mut p Parser) stmt(is_top_level bool) ast.Stmt {
 		.semicolon {
 			return p.semicolon_stmt()
 		}
+		// Allow struct definitions inside functions
+		.key_struct, .key_union {
+			return p.struct_decl(false)
+		}
 		// literals, 'if', etc. in here
 		else {
 			return p.parse_multi_expr(is_top_level)