File structure code completion support for Compose attributes

[rcjsuen]

This builds on top of the work that was introduced in #376.

Services:

• context attribute under build (directories only)
• dockerfile attribute under build
• file attribute under extends
• env_file string attribute
• env_file array item attribute as a string
• env_file array item object's path attribute
• label_file string attribute
• label_file array item attribute as a string
• file attribute under credential_spec
• volumes array item for ./host/folder:/container/folder mounting (implemented in Suggest local files and folders when editing volume paths #376)
• source attribute under the volumes array item if it is a bind mount (implemented in Suggest local files and folders when editing volume paths #376)

Configs:

• file attribute

Secrets:

• file attribute

Include:

• string array item
• path attribute
• path array items
• env_file attribute
• env_file array items

[rcjsuen]

Last bit completed by #408. Closing...