Include `.rs.template` in Rust syntax

## Preliminary Steps

Please confirm you have...
- [x] reviewed [How Linguist Works](https://github.com/github/linguist#how-linguist-works),
- [x] reviewed the [Troubleshooting](https://github.com/github/linguist#troubleshooting) docs,
- [ ] considered implementing an [override](https://github.com/github/linguist#overrides),
- [x] verified an issue has not already been logged for your issue ([linguist issues](https://github.com/issues?utf8=%E2%9C%93&q=is%3Aissue+repo%3Agithub/linguist)).

## Problem Description

Rust template files don't show with the Rust syntax.

### URL of the affected repository:

Example [here](https://github.com/intellij-rust/intellij-rust/pull/6414/files)

### Last modified on:

2020-11-19

### Expected language:

Rust

### Detected language:

None

Start with the How Linguist Works and Troubleshooting documentation, then inspect the language-detection rules used for the Rust language. Use the linked intellij-rust example to reproduce the behavior; done means files ending in .rs.template are detected and displayed with Rust syntax.

Written by the indexing model from the issue text.