Write a function `is_majority` that determines if a given element is the majority element in a sorted array. The majority element is defined as the element that appears more than `n/2` times in the array, where `n` is the length of the array.
#### Function Signature
```python [main.nopy]
def is_majority(arr: list[int], x: int) -> bool:
```
#### Input
- `arr`: A sorted list of integers.
- `x`: An integer, the element to check for majority.
#### Output
- Returns `True` if `x` is the majority element, otherwise `False`.
#### Example Usage
```python [main.nopy]
is_majority([1, 2, 3, 3, 3, 3, 10], 3) # Output: True
is_majority([1, 1, 2, 4, 4, 4, 6, 6], 4) # Output: False
is_majority([1, 1, 1, 2, 2], 1) # Output: True
is_majority([1, 1, 2, 2], 4, 1) # Output: False
```
#### Constraints
- The array is sorted in non-decreasing order.
- The function should use binary search for efficiency.