Add pipe append and inline pipe append

[chiefMarlin]

This PR introduces two new commands:

pipe-append
Takes selected text and pipes it to a shell command.
Appends the shell command's output on a new line immediately after the selected text.

pipe-inline-append
Works similarly but inserts the shell command's output directly at the cursor position, rather than on a new line.

Intent of this is to add flexibility in using shell commands for text completion and insertion.
For example using shell commands to complete code/text.
:pipe-append aichat Given the current context, suggest next line. Output only text, no explanations:
:pipe-append-inline aichat Given the current context, finish this line. Output only text, no explanations:

Adding keybindings for completion

[keys.normal]
"tab" = ":pipe-append aichat \"Given the current context, suggest next line. Output only text, no explanations:\""

[keys.normal]
"tab" = ":pipe-append aichat \"Given the current context, suggest next line. Output only text, no explanations:\""