account for optional dependencies in Prism vendor script

[jpdriver]

• this tweaks the Prism vendor script to additionally account for optional dependencies
  • prompted by fix: load YAML language before Markdown #150, but this should be a more holistic fix
  • other langs may have been broken too 😅
• e.g. in the case of markdown, we were correctly prioritizing markup but not yaml
  • see https://github.com/PrismJS/prism/blob/master/components.json#L873-L879
• n.b. only optional dependencies listed in the includedLangs actually get included though

Screenshots

• (additional console.log temporarily added for illustration)
• note the order the includedLangs are added in

Before

After