Httpsnippet uses ajv but doesn't actually depend on it

In an empty directory:

```bash
$ npm install httpsnippet
$ node
> require('httpsnippet')
Uncaught Error: Cannot find module 'ajv'
```

Used here: https://github.com/Kong/httpsnippet/blob/63650f3f6a921e4a617316ba12f14904d9af4b76/src/helpers/har-validator.ts#L1

Ajv is a dev dependency in this repo (via eslint) but not an actual dependency of the project.
