Add completion for build and ls and update flags

[b-per]

An update to the zsh completion script, based on the flags and subcommands in dbt 1.0.0-b2

The main changes are:

• support for dbt build
• support for dbt ls / dbt list
• support for the -s and --select flags in dbt run, dbt test and dbt build
• update to different flags to align them with the latest version of dbt

I also cleaned the code to remove some functions not used and that could be confusing (I am the person who created the completion file originally but I changed my GitHub name since then)

[joellabes]

Thanks @b-per! Sorry this took so long to merge 💀