Add check to ensure plugin version matches cargo version (#794)

This modifies Rojo's build script to throw a fit if we're building a
plugin with a semver incompatible version. In the process, it moves the
version of the plugin to a file named `Version.txt` that's parsed at
runtime. This should be minimally invasive but it's technically worse
for performance than the hardcoded table and string we had before.

This feels better than a CI check or just manually verifying because it
makes it physically impossible for us to forget since Rojo won't build
with it being wrong.

Cargo.toml

@@ -102,6 +102,7 @@ anyhow = "1.0.44"
 bincode = "1.3.3"
 fs-err = "2.6.0"
 maplit = "1.0.2"
+semver = "1.0.19"
 
 [dev-dependencies]
 rojo-insta-ext = { path = "crates/rojo-insta-ext" }