Support custom word boundaries

[diminutivesloop]

General information

Please report the following information as possible as you

• Terminal program: Terminal 2.12.7
• Operating system: Mac OS 12.6.1
• ZSH framework: oh-my-zsh
• ZSH version: 5.8.1
• ZVM version: 0.9.0

Basic examination

• [x] I have read through the README page
• [x] I have the latest version of zsh-vi-mode
• [ ] I have tested with another terminal program

Problem description

There should be an option to set custom word boundary characters for commands such as w. For example I'd like to add '_' as a word boundary to make editing commands with environment variables easier.