Add support for ERB templates and others

[dan42]

There's an existing parser for <% %> based templates:
https://github.com/tree-sitter/tree-sitter-embedded-template

My main use case is for ERB, but I think it would be very nice to integrate the above in a way that allows any mix of difftastic's supported programming languages with structured text formats.

.html.erb => Ruby embedded in HTML
.xml.erb => Ruby embedded in XML
.json.epy => Python embedded in JSON
.toml.epl => Perl embedded in TOML

Anything goes!