[IDE] Support syntax highlighting for Velocity templates

[StanZGenchev]

Is your feature request related to a problem? Please describe. All Velocity template files (*.html.template, *.js.template, *.extension.template, etc) are treated as plain text files. This makes it difficult to edit the code, have proper indentation, etc.

Describe the solution you'd like HTML templates should have HTML highlighting, JavaScript templates should have JS highlighting, etc.