fix(#749): handle negative step values

[garethbowen]

Closes #749

I have verified this PR works in these browsers (latest versions):

• Chrome
• Firefox
• Safari (macOS)
• Safari (iOS)
• Chrome for Android
• Not applicable

What else has been done to verify that this works as intended?

Test written

Why is this the best possible solution? Were any other approaches considered?

This would've been much easier to do in the vue component but because it's a spec feature I think it's better to have it in the engine for all clients to benefit from.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Do we need any specific form for testing your changes? If so, please attach one.

<h:html xmlns="http://www.w3.org/2002/xforms"
  xmlns:h="http://www.w3.org/1999/xhtml">

  <h:head>
    <h:title>Range controls</h:title>
    <model>
      <instance>
        <root id="range-controls">
          <range-decimal>1.0</range-decimal>
          <range-int>2</range-int>

          <vertical>
            <v-decimal>1.25</v-decimal>
            <v-int>1</v-int>
          </vertical>
        </root>
      </instance>
      <bind nodeset="/root/range-decimal" type="xsd:decimal" />
      <bind nodeset="/root/range-int" type="int" />
      <bind nodeset="/root/vertical/v-decimal" type="decimal" />
      <bind nodeset="/root/vertical/v-int" type="xsd:int" />
    </model>
  </h:head>

  <h:body>
    <range ref="/root/range-decimal" start="-2.0" end="2.0" step="-0.5">
      <label>Range control (decimal)</label>
    </range>
    <range ref="/root/range-int" start="-6" end="4" step="-2">
      <label>Range control (int)</label>
    </range>

    <group ref="/root/vertical">
      <range ref="/root/vertical/v-decimal" start="-5.0" end="7.0" step="-0.5"
        appearance="vertical">
        <label>Range control (decimal)</label>
      </range>
      <range ref="/root/vertical/v-int" start="-3" end="4" step="-3" appearance="vertical no-ticks">
        <label>Range control (int)</label>
      </range>
    </group>
  </h:body>

</h:html>

What's changed

[changeset-bot]

🦋 Changeset detected

Latest commit: 2183dc2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@getodk/xforms-engine	Patch
@getodk/common	Patch
@getodk/xpath	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[latin-panda]

One small comment below, but approving now to unblock

[latin-panda]

I couldn't comment inline 🤷🏽‍♀️ pasting it here instead:

Line 87: The flag looks odd to me. I'd prefer parsing here.

		const step = parseNumericStringAttribute(element, 'step');
		const absStep = (step.startsWith('-') ? step.substring(1) : step) as NumericString; 

return new this(start, end, absStep);

[garethbowen]

The flag looks odd to me. I'd prefer parsing here.

Yeah me too... but I also didn't like the cast. I've created a second function, which calls the assertion after the - has been split off which feels safer to me.