feat(medusa-dev-cli): adds helpers to manage feature flags (#1770)

**Usage**

**Create a new feature flag**
```
$ medusa-dev ff create [name of flag] -d [description of what flag is for]
```
Will put a new file in `packages/medusa/src/loaders/feature-flags/[kebab-cased-flag-name].ts` and fill out the details.

**List feature flags**
```
$ medusa-dev ff list
```
Note: your Medusa repo must be built for the flags to show up

**Delete a feature flag**
```
$ medusa-dev ff delete [name of flag]
```
Will delete a file at `packages/medusa/src/loaders/feature-flags/[kebab-cased-flag-name].ts` if it exists.

packages/medusa-dev-cli/src/feature-flags/template.js

@@ -0,0 +1,13 @@
+export const featureFlagTemplate = ({
+  key,
+  description,
+  defaultValue,
+  envKey,
+}) => {
+  return `export default {
+  key: "${key}",
+  description: "${description}",
+  default_value: ${defaultValue},
+  env_key: "${envKey}",
+}`
+}