refactor: Create shorten package

[gabe565]

Description

Refactor codebase to add a shorten package. This allows other projects to use this library and closes #34.

Some notes:

• Sometimes a dir called pkg is preferred for things that should be public. I could easily move to pkg/shorten. I opted to keep the imports shorter, but let me know if you have a different preference!
• Most of the funcs in the shorten package could be moved to their own packages in internal. Again, I don't mind doing the refactoring if you'd prefer that.
• To simplify usage in other projects, I have added shorten.DefaultConfig() which returns a shorten.Config struct that matches the default flag values. I have also changed shorten.New() so that config is an optional parameter. If not passed, the default config is used.
• I renamed most funcs and vars to make them private. Only shorten.New(), shorten.DefaultConfig(), and Shortener.Shorten() are exposed.
• All tests are passing.

Unfortunately, this PR required some major refactoring and will create merge conflicts with any other open PRs.

Let me know if you have any suggestions!

Example: Usage in another project

main.go

package main

import (
	"fmt"
	"github.com/segmentio/golines/shorten"
)

var code = `package main

var myMap = map[string]string{"key1": "value1", "key2": "value2", "key3": "value3", "key4": "value4", "key5", "value5"}
`


func main() {
	shortener := shorten.New()

	out, err := shortener.Shorten([]byte(code))
	if err != nil {
		panic(err)
	}

	fmt.Print(string(out))
}

Todo

• Add a quick section to the readme that shows usage from other packages.

[mdwhatcott]

I was just looking for a code-shortener-as-a-library and found this project, which is just what I was hoping for! My usecase includes code generation and I was hoping for a library rather than a command. Anything holding up this PR from being merged?

[yipal]

I would also love to see this merged.

[gabe565]

This repo seems pretty inactive, but I can rebase this PR if there's a chance it'll be merged! It might be worth it to investigate gofumpt at this point. Looks like it'll split long lines if the env GOFUMPT_SPLIT_LONG_LINES=on is set