Fix #585 FileTypeUtil should identify Rust files

[bobbylight]

Like it says on the tin. Currently, this returns SyntaxConstants.SYNTAX_STYLE_NONE:

String style = FileTypeUtil.get().guessContentType(new File("test.rs"));

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 80.35%. Comparing base (de0604f) to head (c8910cd). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #586 +/- ## ============================================ + Coverage 80.34% 80.35% +0.01% - Complexity 7781 7786 +5 ============================================ Files 178 178 Lines 30247 30250 +3 Branches 3911 3911 ============================================ + Hits 24301 24308 +7 + Misses 4273 4272 -1 + Partials 1673 1670 -3 ```

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