Move single-trailing-newline extension to packages folder

src/packages/single-trailing-newline/single-trailing-newline/index.coffee

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+module.exports = require 'single-trailing-newline/src/single-trailing-newline'

src/packages/single-trailing-newline/single-trailing-newline/spec/single-trailing-newline-spec.coffee

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+SingleTrailingNewline = require 'single-trailing-newline'
+RootView = require 'root-view'
+fs = require 'fs'
+
+describe "SingleTrailingNewline", ->
+  [rootView, editor, path] = []
+
+  beforeEach ->
+    path = "/tmp/atom-whitespace.txt"
+    fs.write(path, "")
+    rootView = new RootView(path)
+
+    SingleTrailingNewline.activate(rootView)
+    rootView.focus()
+    editor = rootView.getActiveEditor()
+
+  afterEach ->
+    fs.remove(path) if fs.exists(path)
+    rootView.remove()
+
+  it "adds a trailing newline when there is no trailing newline", ->
+    editor.insertText "foo"
+    editor.save()
+    expect(editor.getText()).toBe "foo\n"
+
+  it "removes extra trailing newlines and only keeps one", ->
+    editor.insertText "foo\n\n\n\n"
+    editor.save()
+    expect(editor.getText()).toBe "foo\n"
+
+  it "leaves a buffer with a single trailing newline untouched", ->
+    editor.insertText "foo\nbar\n"
+    editor.save()
+    expect(editor.getText()).toBe "foo\nbar\n"
+
+  it "leaves an empty buffer untouched", ->
+    editor.insertText ""
+    editor.save()
+    expect(editor.getText()).toBe ""

src/packages/single-trailing-newline/single-trailing-newline/src/single-trailing-newline.coffee

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+module.exports =
+  name: "Add a single trailing newline"
+
+  activate: (rootView) ->
+    for buffer in rootView.project.getBuffers()
+      @addSingleTrailingNewlineBeforeSave(buffer)
+
+    rootView.project.on 'new-buffer', (buffer) =>
+      @addSingleTrailingNewlineBeforeSave(buffer)
+
+  addSingleTrailingNewlineBeforeSave: (buffer) ->
+    buffer.on 'before-save', ->
+      if buffer.getLastLine() is ''
+        row = buffer.getLastRow()
+        while row and buffer.lineForRow(--row) is ''
+          buffer.deleteRow(row)
+      else
+        buffer.append('\n')