Fix the way to parse the example

[kceiw]

Description

When we receive the examples from the service, we need to parse them to understand the command names and parameter names etc. It wasn't properly able to parse the examples with complex syntax, for example, the example with "()" or "{}" in them.

The fix is to use a different api to get the CommandAst from the text. That can better parse the examples.

Checklist

• Check this box to confirm: I have read the Submitting Changes section of CONTRIBUTING.md and reviewed the following information:

• SHOULD select appropriate branch. Cmdlets from Autorest.PowerShell should go to generation branch.
• SHOULD make the title of PR clear and informative, and in the present imperative tense.
• SHOULD update ChangeLog.md file(s) appropriately
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header in the past tense. Add changelog in description section if PR goes into generation branch.
  • Should not change ChangeLog.md if no new release is required, such as fixing test case only.
• SHOULD have approved design review for the changes in this repository (Microsoft internal only) with following situations
  • Create new module from scratch
  • Create new resource types which are not easy to conform to Azure PowerShell Design Guidelines
  • Create new resource type which name doesn't use module name as prefix
  • Have design question before implementation
• SHOULD regenerate markdown help files if there is cmdlet API change. Instruction
• SHOULD have proper test coverage for changes in pull request.
• SHOULD NOT introduce breaking changes in Az minor release except preview version.
• SHOULD NOT adjust version of module manually in pull request

[azure-client-tools-bot-prd]

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