installing rustc/cargo for mybinder demo

when we switch to teh rust compiler by default, mybinder stop working, as reoported in #1054 this is because the binder docker image does not have a rust compiler or tools, this install them by using the apt.txt file

.binder/README.md

@@ -0,0 +1,4 @@
+Using mybinder
+-------------
+
+This repository needs some special packages when running examples and tutorial using [mybinder](https://mybinder.org/v2/gh/instagram/LibCST/main?filepath=docs/source/tutorial.ipynb). We use the file apt.txt to install rustc and cargo.

.binder/apt.txt

@@ -0,0 +1,2 @@
+rustc
+cargo

.gitignore

@@ -20,3 +20,4 @@ libcst/_version.py
 target/
 venv/
 .venv/
+.idea/