Emit `MagicCommand` token when it is the assignment value[^1] i.e., on
the right side of an assignment statement.
Examples:
```python
pwd = !pwd
foo = %timeit a = b
bar = %timeit a % 3
baz = %matplotlib \
inline"
```
[^1]: Only `%` and `!` are valid in that position, other magic kinds are
not valid
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