Add a renameproj Command to Rename Project Tags

[AgumasD]

Description:
Currently, renaming a project tag (e.g., +project) in todo.txt-cli is not straightforward. Users need to manually edit the todo.txt file or use repetitive commands to update each instance of a project name. This can be time-consuming and error-prone, especially for users managing multiple tasks under a single project.

A dedicated renameproj command would allow users to rename project tags easily and consistently across all tasks.

Proposed Solution:
Add a renameproj action to the CLI.
The command should work as follows:
todo.sh renameproj +oldproject +newproject
Automatically replace all instances of +oldproject with +newproject in the todo.txt file.
Provide a summary of changes made, e.g., "Renamed +oldproject to +newproject in X tasks."
Benefits:
Streamlines the process of updating project tags.
Reduces manual edits and potential inconsistencies.
Enhances task management efficiency, especially for users managing large projects.

[inkarkat]

Should have a renamecon for consistency, too. And I would prefer to have a generic global substitution command (a la my gsubst add-on-action), and then implement rename{con,proj} in terms of it.