The Mustache highlighting has been modified, so as to also highlight template files and make it more complete (previously, this highlighter was only used within HTML highlighting, in the <script> tag).
I have taken as reference the Mustache highlight files for Vim [1] and Atom [2], and the documentation of Musatche and Handlebars.

Changes:

• Fix and simplify code: within <script>, in normal HTML files, it's not necessary to highlight JavaScript code, so that part is removed.
• Highlight full HTML files, such as Mustache templates. That is, the highlighter is now not hidden, but it's applied to Mustache/Handlebars/Ractive template files.
• Rename: MustacheJS -> Mustache/Handlebars (HTML)
• Improve support for Handlebars [3].
• In 'html.xml', also highlight HTML templates in the <script> tags (with type="text/html") [4].

Sources:

• [1] https://github.com/mustache/vim-mustache-handlebars
• [2] https://github.com/atom/language-mustache
• [3] https://handlebarsjs.com/
• [4] https://stackoverflow.com/questions/5679220/how-does-html-tags-work-inside-script-tag