restore old Alert api

[mayank99]

Changes

similar to #1502, brought back the old Alert api, reverting the breaking change from #1247.

the following two snippets are now equivalent:

<Alert
  type='positive'
  onClose={onClose}
  clickableText='Click me'
  clickableTextProps={{ href: '/' }}
>
  This is an alert.
</Alert>

<Alert.Wrapper type='positive'>
  <Alert.Icon />
  <Alert.Message>
    This is an alert.
    <Alert.Action href='/'>Click me</Alert.Action>
  </Alert.Message>
  <Alert.CloseButton onClick={onClose} />
</Alert.Wrapper>

Other notes:

• added missing ButtonBase (from make disabled buttons interactable #1461) to Alert.Action
• the clickableText, clickableTextProps and onClose props have been brought back but are still deprecated to encourage use of new api.
  • the purpose of restoring the old api in this case is just to make migration easier. clickableText is not a good api so it should be avoided.

Testing

Tested in playground, and added one unit test for legacy api.

Docs

Updated old changeset and old examples in website. Didn't add legacy api example to docs, to encourage new api.

Will update migration guide.