feat: add version storage var in PRBProxyFactory

contracts/IPRBProxyFactory.sol

@@ -26,6 +26,10 @@ interface IPRBProxyFactory {
     /// @param proxy The address of the proxy to make the check for.
     function isProxy(address proxy) external view returns (bool result);
 
+    /// @notice The release version of PRBProxy.
+    /// @dev This is stored in the factory rather than the proxy to save gas for end users.
+    function version() external view returns (uint256);
+
     /// PUBLIC NON-CONSTANT FUNCTIONS ///
 
     /// @notice Deploys a new proxy via CREATE2.

contracts/PRBProxyFactory.sol

@@ -10,6 +10,9 @@ import "./PRBProxy.sol";
 contract PRBProxyFactory is IPRBProxyFactory {
     /// PUBLIC STORAGE ///
 
+    /// @inheritdoc IPRBProxyFactory
+    uint256 public constant version = 1;
+
     /// INTERNAL STORAGE ///
 
     /// @dev Internal mapping to track all deployed proxies.

test/contracts/prbProxyFactory/PRBProxyFactory.behavior.ts

@@ -1,7 +1,12 @@
 import { shouldBehaveLikeDeploy } from "./effects/deploy";
 import { shouldBehaveLikeDeployFor } from "./effects/deployFor";
+import { shouldBehaveLikeVersionGetter } from "./view/version";
 
 export function shouldBehaveLikePrbProxyFactory(): void {
+  describe("View Functions", function () {
+    shouldBehaveLikeVersionGetter();
+  });
+
   describe("Effects Functions", function () {
     describe("deploy", function () {
       shouldBehaveLikeDeploy();

test/contracts/prbProxyFactory/view/version.ts

@@ -0,0 +1,10 @@
+import type { BigNumber } from "@ethersproject/bignumber";
+import { One } from "@ethersproject/constants";
+import { expect } from "chai";
+
+export function shouldBehaveLikeVersionGetter(): void {
+  it("returns the correct version", async function () {
+    const version: BigNumber = await this.contracts.prbProxyFactory.version();
+    expect(One).to.equal(version);
+  });
+}