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.

plugin/src/Config.lua

@@ -2,11 +2,23 @@ local strict = require(script.Parent.strict)
 
 local isDevBuild = script.Parent.Parent:FindFirstChild("ROJO_DEV_BUILD") ~= nil
 
+local Version = script.Parent.Parent.Version
+local realVersion = Version.Value:split(".")
+
+for i = 1, 3 do
+	local num = tonumber(realVersion[i])
+	if num then
+		realVersion[i] = num
+	else
+		error(("invalid version `%s` (field %d)"):format(realVersion[i], i))
+	end
+end
+
 return strict("Config", {
 	isDevBuild = isDevBuild,
 	codename = "Epiphany",
-	version = { 7, 3, 0 },
-	expectedServerVersionString = "7.2 or newer",
+	version = realVersion,
+	expectedServerVersionString = ("%d.%d or newer"):format(realVersion[1], realVersion[2]),
 	protocolVersion = 4,
 	defaultHost = "localhost",
 	defaultPort = "34872",