fix: insert not replace for insert at cursor

[abeatrix]

Reported by @toolmantim, current insert at cursor button uses replace instead of insert

fix: Use insert instead of replace for insert at cursor button

• Change edit.replace to edit.insert for code block insertion button
• Insert completion text at selection start instead of replacing by range

Test plan

• start cody from this branch
• use the insert at cursor button on code block

Before

current selection will be replaced by code block insertion on button click

After

code block will be inserted at the start of the current cursor, and current selection will not be replaced