feat(whiskers): add read_file function

[uncenter]

Adds a read_file function for reading the raw contents of files into templates. Workaround for usecases which would usually involve the include builtin but we cannot use it as a) it requires static paths (e.g. you can do {% include "abc.tera" %} but not {% include flavor.identifier ~ ".tera" %}) and b) it preloads all template paths provided by the glob which doesn't allow for the various encodings Whiskers itself supports. EDIT: Also useful for places where including the contents of the LICENSE file is needed.

[nullishamy]

Code seems fine, though I'm thinking it might be more predictable to go from the path of the tera file itself? Thoughts?

[uncenter]

Code seems fine, though I'm thinking it might be more predictable to go from the path of the tera file itself? Thoughts?

What do you mean? That is what it does. It uses the parent of the Whiskers template as the root for resolving paths.

[nullishamy]

Usage of cwd in the code would imply to me that it goes from the invocation directory, maybe some docs / rename to make this clearer? Upon closer inspection it seems it does what I'd want it to do, so no qualms about the implementation as such now!

[uncenter]

Yay! Figured out why tests were failing - turns out whitespace control is undocumented for usage in expressions, but you can do it (Keats/tera#919)!