fix Yul stack assignments operator (NomicFoundation#1062)

This statement uses `=:` and `= :` operators, with no LHS.

More details: https://docs.soliditylang.org/en/v0.4.26/assembly.html

Closes NomicFoundation#1060

.changeset/itchy-candles-remain.md

@@ -0,0 +1,5 @@
+---
+"@nomicfoundation/slang": patch
+---
+
+fix stack assignments operator `=:`.

crates/solidity/inputs/language/src/definition.rs

@@ -1906,6 +1906,13 @@ codegen_language_macros::compile!(Language(
                             name = Equal,
                             definitions = [TokenDefinition(scanner = Atom("="))]
                         ),
+                        Token(
+                            name = EqualColon,
+                            definitions = [TokenDefinition(
+                                enabled = Till("0.5.0"),
+                                scanner = Atom("=:")
+                            )]
+                        ),
                         Token(
                             name = EqualEqual,
                             definitions = [TokenDefinition(scanner = Atom("=="))]
@@ -4235,25 +4242,38 @@ codegen_language_macros::compile!(Language(
                                 expression = Required(YulExpression)
                             )
                         ),
+                        Enum(
+                            name = YulAssignmentOperator,
+                            variants = [
+                                EnumVariant(reference = YulColonAndEqual, enabled = Till("0.5.5")),
+                                EnumVariant(reference = ColonEqual)
+                            ]
+                        ),
+                        Struct(
+                            name = YulColonAndEqual,
+                            enabled = Till("0.5.5"),
+                            fields = (colon = Required(Colon), equal = Required(Equal))
+                        ),
                         Struct(
                             name = YulStackAssignmentStatement,
                             enabled = Till("0.5.0"),
                             fields = (
-                                assignment = Required(YulAssignmentOperator),
-                                expression = Required(YulExpression)
+                                assignment = Required(YulStackAssignmentOperator),
+                                variable = Required(YulIdentifier)
                             )
                         ),
                         Enum(
-                            name = YulAssignmentOperator,
+                            name = YulStackAssignmentOperator,
+                            enabled = Till("0.5.0"),
                             variants = [
-                                EnumVariant(reference = YulColonEqual, enabled = Till("0.5.5")),
-                                EnumVariant(reference = ColonEqual)
+                                EnumVariant(reference = YulEqualAndColon),
+                                EnumVariant(reference = EqualColon)
                             ]
                         ),
                         Struct(
-                            name = YulColonEqual,
-                            enabled = Till("0.5.5"),
-                            fields = (colon = Required(Colon), equal = Required(Equal))
+                            name = YulEqualAndColon,
+                            enabled = Till("0.5.0"),
+                            fields = (equal = Required(Equal), colon = Required(Colon))
                         ),
                         Struct(
                             name = YulIfStatement,