feat: add env init command that copies from template placeholders

[zimeg]

Changelog

A new env init command copies a template placeholder ".env.sample" or ".env.example" file to the ".env" file of a project.

Summary

This PR adds the env init command that copies from template placeholders 🌲

Preview

Included are expected cases and common edges to encounter! 🔬 ✨

⚡ Bolt Frameworks

1. An existing placeholder file is used for the new ".env" file
2. An existing ".env" file is not overwritten
3. A missing placeholder file notes to use "env set" command

🥀 ROSI Features

1. Deployed apps recommend the "env set" command
2. Local development requires selection and saves to project file

Testing

Checkout the changes of this branch and experiment with either a Bolt or Deno app:

$ slack create asdf
$ cd asdf
$ slack install     # Required for Deno - Optional for Bolt
$ cat .env.sample   # Inspect values upcoming
$ slack env init
$ cat .env
$ slack env init    # Reattempt the initilization

Notes

Some follow up remains proposed to improve this but is left out of scope for this PR 🏁

• Prompting for values included in placeholders might be a meaningful improvement to avoid unexpected values. We might find the following format interesting toward this but without much exploration right now:

# COMMENTED_VARIABLE=       # Not prompted
UNCOMMENTED_VARIABLE=       # Prompted
SET_VARIABLE=debug_example  # Not prompted

• Including this command as part of project init ought make setup a breeze it's been said 😉

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

❌ Patch coverage is 89.71963% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.38%. Comparing base (783ac1a) to head (5f997fb).

Files with missing lines	Patch %	Lines
internal/slackdotenv/slackdotenv.go	68.18%	5 Missing and 2 partials ⚠️
cmd/env/init.go	95.06%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #516      +/-   ##
==========================================
+ Coverage   71.25%   71.38%   +0.13%     
==========================================
  Files         222      223       +1     
  Lines       18682    18789     +107     
==========================================
+ Hits        13311    13413     +102     
- Misses       4190     4195       +5     
  Partials     1181     1181              

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[mwbrooks]

🙇🏻 Thanks a bunch for opening this PR! 🎉 I'm excited to see this one landing.

🧠 Could we swap "template placeholders" for "template file" or ".env template file"? "Placeholder" tends to imply a variable or token that gets substituted, which is a bit ambiguous here. Since we're copying the file verbatim, ".env template file" is more precise and the common convention in the wild from a little Claude research.

[mwbrooks]

suggestion: Minor suggestion to clarify that we're initializing from a file (plus it's shorter, haha).

Suggested change

	Meaning: "Initialize environment variables from template placeholders",
	Meaning: "Initialize environment variables from a template file",