fix(cli): Fix plugin loading with rewrite of loading logic (#9171)

**Problem**
Although the cli plugin loading behaviour was functional for our
storybook and data-migrate needs, some flaws where highlighted when it
came to integrate third party packages. The loading logic was somewhat
scatterbrained and had no tests.

**Changes**
1. Reworks the plugin loading logic and, hopefully, makes the logic
easier to follow.
2. Adds tests for the loading logic. This will help catch unintentional
changes in behaviour in future updates.
3. I removed the `version` option from the toml config for experimental
cli plugins. If users wish to enforce a specific version they should
install such a version in the standard way they would for any third
party package - using the package.json and yarn.

**Notes**
1. This is still considered an "experimental" feature so we can
potentially be more loose - which is why I felt a larger rewrite was
advantageous in this case.

packages/cli-helpers/src/lib/__tests__/version.test.ts

@@ -261,14 +261,6 @@ describe('version compatibility detection', () => {
     )
   })
 
-  test('throws if no redwoodjs engine is found', async () => {
-    await expect(
-      getCompatibilityData('@scope/package-name', '0.0.1')
-    ).rejects.toThrowErrorMatchingInlineSnapshot(
-      `"The package '@scope/package-name' does not specify a RedwoodJS compatibility version/range"`
-    )
-  })
-
   test('throws if no latest version could be found', async () => {
     jest.spyOn(global, 'fetch').mockImplementation(() => {
       return {

packages/cli-helpers/src/lib/version.ts

@@ -67,18 +67,15 @@ export async function getCompatibilityData(
     ? packument['dist-tags'][preferredVersionOrTag]
     : preferredVersionOrTag
 
-  // Does that version of the package support the current version of RedwoodJS?
+  // Extract the package's redwoodjs engine specification for the preferred version
   const packageRedwoodSpecification =
     packument.versions[preferredVersion].engines?.redwoodjs
 
-  if (packageRedwoodSpecification === undefined) {
-    throw new Error(
-      `The package '${packageName}' does not specify a RedwoodJS compatibility version/range`
-    )
-  }
-
   // We have to use the semver.intersects function because the package's redwoodjs engine could be a range
-  if (semver.intersects(projectRedwoodVersion, packageRedwoodSpecification)) {
+  if (
+    packageRedwoodSpecification !== undefined &&
+    semver.intersects(projectRedwoodVersion, packageRedwoodSpecification)
+  ) {
     const tag = getCorrespondingTag(preferredVersion, packument['dist-tags'])
     return {
       preferred: {