feat!: Collapse all yank-join commands into a single :yank-join with flags

[nik-rev]

Instead of 3 similar commands, we now have 1 command :yank-join:

┌────────────────────────────────────────────────────────────────────────────────────────┐
│ Yank the selections joined with a separator                                            │
│ Aliases: yj                                                                            │
│ Flags:                                                                                 │
│   --separator/-s <arg>  Separator to between joined selections [default: newline]      │
│   --register/-r <arg>   Yank into this register                                        │
└────────────────────────────────────────────────────────────────────────────────────────┘
:yank-join

Additionally:

• Added alias yj for yank-join
• Added tests for each flag + without

Breaking changes:

• Removed :clipboard-yank-join, use yank-join --register + instead
• Removed :primary-clipboard-yank-join, use yank-join --register * instead