fix: Support the constant state modifier in function types

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

@@ -2696,6 +2696,7 @@ codegen_language_macros::compile!(Language(
                                 EnumVariant(reference = ExternalKeyword),
                                 EnumVariant(reference = PrivateKeyword),
                                 EnumVariant(reference = PublicKeyword),
+                                EnumVariant(reference = ConstantKeyword, enabled = Till("0.5.0")),
                                 EnumVariant(reference = PureKeyword, enabled = From("0.4.16")),
                                 EnumVariant(reference = ViewKeyword, enabled = From("0.4.16")),
                                 EnumVariant(reference = PayableKeyword)

crates/solidity/testing/snapshots/cst_output/FunctionType/constant_state_mutability/generated/0.4.11-success.yml

@@ -0,0 +1,32 @@
+# This file is generated automatically by infrastructure scripts. Please don't edit by hand.
+
+Source: >
+  1  │ function () constant internal returns (uint)                                     │ 0..44
+
+Errors: []
+
+Tree:
+  - (FunctionType): # "function () constant internal returns (uint)\n" (0..45)
+      - (function_keyword꞉ FunctionKeyword): "function" # (0..8)
+      - (parameters꞉ ParametersDeclaration): # " ()" (8..11)
+          - (leading_trivia꞉ Whitespace): " " # (8..9)
+          - (open_paren꞉ OpenParen): "(" # (9..10)
+          - (parameters꞉ Parameters): [] # (10..10)
+          - (close_paren꞉ CloseParen): ")" # (10..11)
+      - (attributes꞉ FunctionTypeAttributes): # " constant internal" (11..29)
+          - (item꞉ FunctionTypeAttribute): # " constant" (11..20)
+              - (leading_trivia꞉ Whitespace): " " # (11..12)
+              - (variant꞉ ConstantKeyword): "constant" # (12..20)
+          - (item꞉ FunctionTypeAttribute): # " internal" (20..29)
+              - (leading_trivia꞉ Whitespace): " " # (20..21)
+              - (variant꞉ InternalKeyword): "internal" # (21..29)
+      - (returns꞉ ReturnsDeclaration): # " returns (uint)\n" (29..45)
+          - (leading_trivia꞉ Whitespace): " " # (29..30)
+          - (returns_keyword꞉ ReturnsKeyword): "returns" # (30..37)
+          - (variables꞉ ParametersDeclaration): # " (uint)\n" (37..45)
+              - (leading_trivia꞉ Whitespace): " " # (37..38)
+              - (open_paren꞉ OpenParen): "(" # (38..39)
+              - (parameters꞉ Parameters): # "uint" (39..43)
+                  - (item꞉ Parameter) ► (type_name꞉ TypeName) ► (variant꞉ ElementaryType) ► (variant꞉ UintKeyword): "uint" # (39..43)
+              - (close_paren꞉ CloseParen): ")" # (43..44)
+              - (trailing_trivia꞉ EndOfLine): "\n" # (44..45)