text: Title: Support item assignment with lists and dicts like `${x}[key] =    Keyword`
  Body: These aren't currently supported:
  ```
  ${dict} =    Create Dictionary    key=initial
  ${dict}[key] =    Set Variable    new
  
  ${list} =    Create List    one    two
  ${list}[0] =    Set Variable    new
  ```
  Especially setting dictionary items would be useful, but there certainly are cases where setting list items is convenient as well. In fact, this syntax should work with any object that has `__setitem__`.
  
  Things to take into account:
  - It should be possible to specify the key/index as a variable like `${x}[${y}]`.
  - It should be possible to give list indices as strings and as integers.
  - Lists should support slices.
  - Error message should be clear if the item is immutable (tuple, string, immutable mapping, ...).
  
  This would be a convenient feature, but because it's possible to manipulate lists and dicts with Collections keywords I don't consider this high priority. See also #4545 that proposes supporting `${x${y}} =  Keyword` and also contained this enhancement in it initially.