feat: add scaffold completion feature

[yaegassy]

Description

• This completion feature will only work on the first line of the file.
• Typing class_scaffold, interface_scaffold, trait_scaffold or enum_scaffold will output completion suggestions.
• Inserts a class name based on the file name
• Inserts a namespace based on the composer configuration
  • No namespace insertion is performed for files for which namespace cannot be resolved

DEMO (mp4)

coc-intelephense-scaffold-completion.mp4