[github config] Use Rust syntax highlighting for Move

We haven't built Linguist syntax highlighting for Move yet. Using Rust syntax highlighting will be more readable than no highlighting at all.

Closes: #3039
Approved by: bmwill

.gitattributes

@@ -10,3 +10,7 @@
 *.toml text
 *.txt text
 *.yml text
+
+# Use Rust syntax highlighter for Move and Move IR code
+*.move linguist-language=Rust
+*.mvir linguist-language=Rust