IAM policy JSON Validator

## Problem
In VS Code, most of my problems with IAM Policies is small syntax errors on my part. I have Parkinsons and mistyping is common occurrence. I typically live in visual studio code for all of my day-to-day tasks, if possible.

## Expected behavior

As such it would be nice to have support IAM Validation syntax/validation checker, like what is online with errors and warnings on a given IAM Policy.
Example:
```
{
"Version": "2012-10-17",
"Statement": [
{
"Sid": "AccessToViewTerraformStateLockandReset",
"Action": [
"dynamodb:ListTables",
"dynamodb:GetItem",
"dynamodb:PutItem",
"dynamodb:UpdateItem",
"dynamodb:DeleteItem",
"asdf:asdf"
],
"Effect": "Allowed",
"Resource": "warn:dynamodb:sometable"
},
]
}
```
The validation would find the following using the above code block.

Errors:

- `asdf:asdf` as it is not a valid action.
- `Allowed` as it should really be `Allow`
- `warn:dynamodb:sometable` as it is not a valid formatted Arn.
- ` },` json syntax error due to misplaced comma.
Warnings:

- `dynamodb:ListTables` cannot be restricted to a specific resource.

It would also be nice to have a way to list the possible IAM Actions for a given service, as right now I get code completion sometimes, but I don't always remember the prefixes for each service.

No implementation files or tests are named. Start with the example IAM policy and inspect the extension's existing completion or validation entry points; done means reporting the requested syntax errors and warnings and offering a way to list possible IAM actions for a service.

Written by the indexing model from the issue text.