JSON, key-value-pairs in groups to "flat" key-value-pair

Each key of a JSON should be combined with _ (or any valid JSON symbol). To have a simple key-value list.

I have the following structure. A few JSON groups (no arrays), inside the groups there are key-value pairs. I need to flat them to a single key-value-list. I tried jq but there is only something "nested" / "unnested". I do not find something about flatten or compaine the keys.

So it should be "key_subkey_subsubkey": "value"

{"welcome": {"title" : "Hello World"    },"block1": {"header": "My Header","body": "My BODY of block 1","footer": "My Footer"    },"multi": {"level-01-A": {"head": "Head Section","foot": "Foot Section""level-02-A": {"head": "Head Section Level 2 A","fead": "Foot Section Level 2 A"            },"level-02-B": {"head": "Head Section Level 2 B","fead": "Foot Section Level 2 B"            },        },"level-01-B": {"head": "Head Section","foot": "Foot Section"        }"no-level" : "Foo Bar",         }}

and I want to have

{"welcome_title" : "Hello World","block1_header": "My Header","block1_body": "My BODY of block 1","block1_footer": "My Footer","multi_level-01-A_head": "Head Section","multi_level-01-A_foot": "Foot Section","multi_level-01-A_level-02-A_head": "Head Section Level 1 A","multi_level-01-A_level-02-A_fead": "Foot Section Level 1 A","multi_level-01-A_level-02-B_head": "Head Section Level 1 B","multi_level-01-A_level-02-B_fead": "Foot Section Level 1 B","multi_level-01-B_head": "Head Section","multi_level-01-B_foot": "Foot Section","multi_no-level" : "Foo Bar"}

Any idea, what tool I can use?