## Changes
While working on the v2 of #1744, I found that:
* Template initialization first copies built-in templates to a temporary
directory before initializing them
* Reading a template's contents goes through a `filer.Filer` but is
hardcoded to a local one
This change updates the interface for reading templates to be `fs.FS`.
This is compatible with the `embed.FS` type for the built-in templates,
so they no longer have to be copied to a temporary directory before
being used.
The alternative is to use a `filer.Filer` throughout, but this would
have required even more plumbing, and we don't need to _read_ templates,
including notebooks, from the workspace filesystem (yet?).
As part of making `template.Materialize` take an `fs.FS` argument, the
logic to match a given argument to a particular built-in template in the
`init` command has moved to sit next to its implementation.
## Tests
Existing tests pass.
## Changes
In a world before this PR, all files would be treated as `go text
templates`, making the content in these files quake in fear since they
would be executed (as a template).
This PR makes it so that only files with the `.tmpl` extension are
understood to be templates. This is useful for avoiding ambiguity in
cases like where a binary file could be interpreted as a go text
template otherwise.
In order to do so, we introduce the `copyFile` struct which does a copy
of the source file from the template without loading it into memory.
## Tests
Unit tests
Pieter Noordhuis
shreyas-goenka