Previously, endless method definitions in assignment contexts like
`x = def f = p 1` would fail to parse because command calls (method
calls without parentheses) were only accepted when the surrounding
binding power was less than `PM_BINDING_POWER_COMPOSITION`.
This fix specifically checks for assignment context and allows command
calls in those cases while maintaining the existing behavior for other
contexts. This ensures that:
- `x = def f = p 1` parses correctly (previously failed)
- `private def f = puts "Hello"` still produces the expected error
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