`merge_table` does not handle merge child table header columns properly

# Issue

The expectation from using `merge_tables` is that it will convert 2 tables like this:
```
Headers:
C1 C2 C3
Rows:
R1
R2

Headers:
R3
Rows:
R4
```

into:

```
Headers:
C1 C2 C3
Rows:
R1
R2
R3
R4
```

But instead the output is

```
Headers:
C1 C2 C3
R3
Rows:
R1
R2
R4
```

This is because the headers are identified based on the existence of `COLUMN_HEADER` in a block's `EntityTypes` field and the child table's top block keep this entity type even after merging. The solution is to simply drop the this value from `EntityTypes` if it's there while merging.

Start at the TypeScript implementation of merge_table/merge_tables and reproduce the two-table example from the issue. Inspect how COLUMN_HEADER in EntityTypes is handled while merging; done means the child table header is treated as a row and the merged output contains C1 C2 C3 followed by R1 through R4.

Written by the indexing model from the issue text.