Generate .env when run shipjs setup

**Is your feature request related to a problem? Please describe.**
When we run `shipjs prepare` at local, .env is needed.
But Shipjs's log when run command `shipjs setup` does not explain that thing.

**Describe the solution you'd like**
I want to propose two ways.

- Generate `.env` when run `npx shipjs setup`

```
#======= Attention: Please don't commit this file =======

# Used when using the `shipjs prepare` command
GITHUB_TOKEN=

```

- Include this text in log when finished `shipjs setup`:
`create .env file on project's root. The file used when run shipjs prepare`

**Describe alternatives you've considered**

**Additional context**
If already exists `.env`, insert text to the file.

Start at the implementation of the `shipjs setup` command and trace how it reports completion and handles project-root files. Done means setup creates the requested `.env` content, or inserts it when `.env` already exists, and communicates the file's purpose in the command output.

Written by the indexing model from the issue text.