feat(schema): Add schema for I-x-x subtype

In (#686), a new instruction schema was created
for types/subtypes. This commit add the schema
for the I-x-x subtype. This commit also add schema
for immediate variable and variable type. It is also
part of effort in (#655).

Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>

Author: Unique-Usman

spec/schemas/inst_var_type_schema.json

@@ -17,7 +17,7 @@
       "type": "string"
     },
     "type": {
-      "enum": ["register_reference"]
+      "enum": ["register_reference", "immediate"]
     }
   },
   "allOf": [

spec/std/isa/inst_subtype/I/I-x-x.yaml

@@ -0,0 +1,21 @@
+# Copyright (c) 2025 Usman Akinyemi
+# SPDX-License-Identifier: BSD-3-Clause-Clear
+
+# yaml-language-server: $schema=../../../schemas/inst_subtype_schema.json
+
+$schema: inst_subtype_schema.json#
+kind: instruction_subtype
+name: I-x-x
+
+data:
+  type: { "$ref": "inst_type/I.yaml#" }
+  subtype: { "$ref": "inst_subtype/I/I-x-x.yaml#" }
+  opcodes:
+    $inherits: inst_type/I.yaml#/opcodes
+  variables:
+    func11:
+      $inherits: inst_var/imm.yaml#/data
+    xs1:
+      $inherits: inst_var/xs1.yaml#/data
+    xd:
+      $inherits: inst_var/xd.yaml#/data

spec/std/isa/inst_var/i-imm12.yaml

@@ -0,0 +1,12 @@
+# Copyright (c) 2025 Usman Akinyemi
+# SPDX-License-Identifier: BSD-3-Clause-Clear
+
+# yaml-language-server: $schema=../../schemas/inst_var_schema.json#
+
+$schema: inst_var_schema.json#
+kind: instruction_variable
+name: imm
+
+data:
+  $inherits: inst_type/I.yaml#/variables/imm
+  type: { "$ref": "inst_var_type/imm.yaml#" }

spec/std/isa/inst_var_type/imm.yaml

@@ -0,0 +1,9 @@
+# Copyright (c) 2025 Usman Akinyemi
+# SPDX-License-Identifier: BSD-3-Clause-Clear
+
+# yaml-language-server: $schema=../../schemas/inst_var_type_schema.json#
+
+$schema: inst_var_type_schema.json#
+kind: instruction_variable_type
+name: imm
+type: immediate