make `cursor_trail` work on the same command prompt only

**Is your feature request related to a problem? Please describe.**

Currently `cursor_trail` always shows cursor movement once the distance is beyond a certain threshold.

However, in most cases when the cursor makes a big movement, my eyes either know where
it will land or don't care. For example,

- After typing in a long command and pressing "Enter", I know it will show up in the next command line prompt.
- When a program outputs long segments of text, I don't care where the cursor is.

However, the unnecessary animation in these scenarios is distractive.

The moments I see cursor_trail is only needed when I move cursor within a prompt
buffer. For example, I may press `Alt-F` to jump a word, and the word has an
unknown length to my eyes until I look carefully.

I tried to adjust `cursor_trail_start_threshold` and `cursor_trail_decay`, but
they don't seem to be able to address the problem.

**Describe the solution you'd like**
Add a new option `cursor_trail_scenario` to specify when to enable cursor
movement.

Start by locating the cursor_trail implementation and the existing cursor_trail_start_threshold and cursor_trail_decay option handling. Define how the requested scenario should distinguish movement within one command prompt from movement caused by submitted commands or program output, then verify the behavior for those examples.

Written by the indexing model from the issue text.