Expect indented case block instead of match stmt

[dhruvmanila]

Summary

This PR adds a new Clause::Case and uses it to parse the body of a case block. Earlier, it was using Match which would give an incorrect error message like:

  |
1 | match subject:
2 |     case 1:
3 |     case 2: ...
  |     ^^^^ Syntax Error: Expected an indented block after `match` statement
  |

Test Plan

Add test case and update the snapshot.

[github-actions]

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