bs-tc does not copy build-icons.js, so the build of a new child theme fails

Problem/Motivation

In bs_base 2.0.x, package.json has "build-icons": "node build-icons.js", and the build script runs build-icons before the CSS build. drush bs-tc copies package.json and .fantasticonrc.js into the new child theme, but not build-icons.js, because that file is missing from the list in BsLibCommands::copyThemeFiles().

As a result, npm run build and drush bs-tb fail in every new child theme with Cannot find module .../build-icons.js. The Sass build then fails on @import "variables/icons", because build-icons.js generates that file. No CSS is written, and the site renders unstyled.

Steps to reproduce

1. drush bs-tc bs_base mytheme "My theme" "My theme"
2. In the new theme folder, run npm run build.
3. The build fails with Cannot find module .../mytheme/build-icons.js, and no css/ folder is written.

Proposed resolution

Add 'build-icons.js', to the $files list in BsLibCommands::copyThemeFiles(), next to .fantasticonrc.js. The file is copied only if it exists in the parent theme, so parent themes without it are not affected.

This is an interim fix. #3628146: Share build-icons.js, the fantasticon config and template, and gulpfile.js code with child themes instead of copying them proposes that child themes use the icon build code and the gulpfile.js code from bs_base instead of copying them. Once that lands, bs-tc needs these changes:

• Stop copying build-icons.js and .fantasticonrc.js.
• Copy only fantasticon/icons.json from the fantasticon folder, and no longer fantasticon/scss.hbs.
• Remove gulpfile.js from the search and replace in BsLibCommands that changes the parent's machine name to the child's. The new gulpfile.js finds bs_base by the name bs_base in gulp-options.yml. For a child theme of bs_base, the replacement would change that name to the child's, and the build would fail.

Remaining tasks

• Add build-icons.js to the copy list.
• Make the follow-up changes above once #3628146: Share build-icons.js, the fantasticon config and template, and gulpfile.js code with child themes instead of copying them lands.

User interface changes

None.

API changes

None.

Data model changes

None.