Fix label ordering

[balderdash]

Trailing labels aren't being rendered correctly under the current custom block parser, e.g.:

  {
    "func": "blahblah",
    "blockText": "before input {INPUT} after input",
    "args": [
      {
        "name": "INPUT"
      }
    ]
  }

Before:

After:

Things get really weird with multi-line blocks:

  {
    "func": "dosomething",
    "blockText": "val 1 {VALUE1} field input {FIELD} val 2 {VALUE2} end",
    "args": [
      {
        "name": "FIELD",
        "field": true
      },
      {
        "name": "VALUE1"
      },
      {
        "name": "VALUE2"
      }
    ],
    "inline": false
  }

Before:

After:

The code change to make this happen was grouping inputs into inputRows and handling each one independently (replacing the broken and ugly getPreviousInput approach I had tried earlier). Each inputRow is just a group of inputs ending with a line-ending input, i.e. a value, statement, or dummy input. The labels and field inputs in each row can now be assembled in a fairly straightforward way:

code-dot-org/apps/src/block_utils.js

Lines 577 to 590 in 33d6e59

	inputRows.forEach(inputRow => {
	// Create the last input in the row first
	const lastInputConfig = inputRow[inputRow.length - 1];
	const lastInput = inputTypes[lastInputConfig.mode]
	.addInputRow(blockly, block, lastInputConfig);
	// Append the rest of the inputs onto that
	inputRow.slice(0, -1).forEach(inputConfig => {
	inputTypes[inputConfig.mode].addInput(blockly, block, inputConfig, lastInput);
	});
	// Finally append the last input's label
	lastInput.appendTitle(lastInputConfig.label);
	});

For inline blocks, the rows all just string together in a long line, looking just like blockText (but a block),

The other bit of refactoring that made sense to do here was adopting the customInputTypes config format for all the input types (this is the thing I added for the location picker input in #21955).

[Hamms]

This is great!

I'd love to see some tests

[Hamms]

nit: could we get a jsdoc @typedef for input definitions?

[Hamms]

nit: could just be lastRow.length

[Hamms]

👍

[balderdash]

There are some (clearly insufficient) unit tests in blockUtilsTest.js, but I'd like to add some kind of block screenshot tests that actually verify that things are rendering the way we want them to.