Add insertNewlineKeepIndent command

FEATURE: The `insertNewlineKeepIndent` command inserts a newline along
with the same indentation as the line before.

Issue codemirror/dev#1370

src/README.md

@@ -187,6 +187,8 @@ with key bindings for a lot of them.
 
 @insertNewlineAndIndent
 
+@insertNewlineKeepIndent
+
 @insertBlankLine
 
 ### Undo History

src/commands.ts

@@ -697,6 +697,19 @@ export const insertNewline: StateCommand = ({state, dispatch}) => {
   return true
 }
 
+/// Replace the selection with a newline and the same amount of
+/// indentation as the line above.
+export const insertNewlineKeepIndent: StateCommand = ({state, dispatch}) => {
+  dispatch(state.update(state.changeByRange(range => {
+    let indent = /^\s*/.exec(state.doc.lineAt(range.from).text)![0]
+    return {
+      changes: {from: range.from, to: range.to, insert: state.lineBreak + indent},
+      range: EditorSelection.cursor(range.from + indent.length + 1)
+    }
+  }), {scrollIntoView: true, userEvent: "input"}))
+  return true
+}
+
 function isBetweenBrackets(state: EditorState, pos: number): {from: number, to: number} | null {
   if (/\(\)|\[\]|\{\}/.test(state.sliceDoc(pos - 1, pos + 1))) return {from: pos, to: pos}
   let context = syntaxTree(state).resolveInner(pos)

test/test-commands.ts

@@ -1,5 +1,6 @@
 import {EditorState, StateCommand, Extension} from "@codemirror/state"
-import {indentMore, indentLess, indentSelection, insertNewlineAndIndent,
+import {indentMore, indentLess, indentSelection,
+        insertNewlineAndIndent, insertNewlineKeepIndent,
         deleteTrailingWhitespace, deleteGroupForward, deleteGroupBackward,
         moveLineUp, moveLineDown} from "@codemirror/commands"
 import {javascriptLanguage} from "@codemirror/lang-javascript"
@@ -66,6 +67,21 @@ describe("commands", () => {
        test("<{\n{\n{\n{}\n}\n}\n}>", "<{\n  {\n    {\n      {}\n    }\n  }\n}>"))
   })
 
+  describe("insertNewlineKeepIndent", () => {
+    function test(from: string, to: string) {
+      ist(stateStr(cmd(mkState(from), insertNewlineKeepIndent)), to)
+    }
+
+    it("keeps indentation", () =>
+      test("  one|", "  one\n  |"))
+
+    it("keeps zero indentation", () =>
+      test("one|two", "one\n|two"))
+
+    it("deletes the selection", () =>
+      test("if x:\n  one<two\n  three>four", "if x:\n  one\n  |four"))
+  })
+
   describe("insertNewlineAndIndent", () => {
     function test(from: string, to: string) {
       ist(stateStr(cmd(mkState(from, javascriptLanguage), insertNewlineAndIndent)), to)