[Feature Request] Allow sl pr submit to Select from Saved GitHub PR Templates

[maruffahmed]

Problem Description

When creating a pull request (PR), having a clear description is really helpful for understanding its context. Currently, in the Sapling tool, when a PR is created, the description is replaced by a stack tree view that shows the PR structure. Although you can manually set a description using GitHub's PR interface, it will get overwritten with the stack tree the next time the sl pr submit command is used.

Feature Description

It would be helpful to add a feature in the sl pr submit command that lets users choose a template for the PR description. This way, the PR description can remain consistent. Additionally, it would be better if the stack tree view could be moved to a comment instead of replacing the description. The first time a user runs the sl pr submit command, they could select a description template. Then, for each subsequent sl pr submit, the stack tree would be updated as a comment, while the chosen description remains unchanged.