Support likert appearance

[florianm]

likert widget https://github.com/getodk/build2xlsform/blob/master/src/convert.ls#L64

[florianm]

Status quo on latest Build/build2xlsform master - kinda works although not explicitly supported.

Build @ getodk/build@bf0f687
build2xlsform @ facbc12

A simple Build form with two metadata fields (device ID and start geopoint) plus a choose one with likert appearance.

Sample forms: likert.zip

• 01 The ODK Build save file
• 02 Export as XML (appearance "likert" - lower case)
• 03 Export as XLSForm (appearance "Likert")
• 04 XLSForm converted with pyxform (xls2xform 03_27-likert.xlsx) and renamed with prefix 04

Warnings:
[row : 3] Question has no label: {'name': 'start_geopoint', 'type': 'start-geopoint'}
Conversion complete!

• 04 passes ODK Validate java -jar odkvalidate.jar 04_27-likert.xml

Loading the direct XLSForm export into ODK Central v1.3 throws a warning about the missing language code.
This issue is tracked at getodk/build#260 but could be solved here in build2xlsform.
Once the labels are corrected from label::English to label::English (en), the form uploads and renders in ODK Collect x2022.1.1 as likert widget.
It seems the "likert" appearance is case insensitive and still works as appearance="Likert" because the XLSForm is converted (by Central's pyxform) and distributed to Collect as an XForm in the case insensitive format XML.

I still want to explicitly support the likert appearance with a translation of label "Likert" to value "likert" so that in future, Build could choose to change the label.