Add autocomplete support for 'outer' keyword

[melnikalex]

Summary

Implements autocomplete support for the outer keyword in the pkl-intellij plugin.

Changes

• New file: ComputeOuterType.kt - Computes the type of outer references by walking up the PSI tree to find the enclosing object scope
• Modified: ComputeExprType.kt - Updated to use computeOuterType() instead of returning Type.Unknown for PklOuterExpr
• Test: Added unit test test complete from outer in CompletionTest.kt

Implementation Details

The computeOuterType() function follows the same pattern as computeThisType():

1. Walks up the PSI tree from the current element
2. Identifies the immediate enclosing object (skip it)
3. Continues to find the next enclosing object
4. Returns the type of that outer object

This enables autocomplete to work when typing outer. inside nested object expressions, showing members of the enclosing scope.

Testing

• All existing tests pass
• New test case validates that outer. shows correct members (methods and properties) from the enclosing object
• Manually tested with the example from the issue

Example Usage

class BirdSpec {
  name: String
  songs: Listing<Song>
  function generateSong(_tune: String): Song = new {
    tune = _tune
  }
}

peacock: BirdSpec = new {
  name = "peacock"
  songs {
    outer.generateSong("beep")  // autocomplete now works\!
  }
}