runtime(handlebars): adds handlebars template syntax & indent support

runtime(handlebars): adds handlebars template syntax & indent support

The runtime had support to detect handlebars (*.hbs) files as filetype
handlebars but was lacking any indent or syntax highlighting for that
filetype.

The handlebars syntax file is also a prerequisite for the glimmer
syntax.

Permission was granted by the original author to retrofit these into the
Vim runtime. Original License (MIT) maintained in code comments.

related: #19569

Signed-off-by: Devin Weaver <suki@tritarget.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>