all: Switch the --lang syntax to use argv instead

It was a bit awkward using `mgmt run lang --lang <input>` so instead, we
now drop the --lang, and read the positional argv for the input.

This also does the same for the --yaml frontend.

examples/lang/env0.mcl

@@ -1,5 +1,5 @@
 # read and print environment variable
-# env TEST=123 EMPTY= ./mgmt run --tmp-prefix --converged-timeout=5 lang --lang=examples/lang/env0.mcl
+# env TEST=123 EMPTY= ./mgmt run --tmp-prefix --converged-timeout=5 lang examples/lang/env0.mcl
 
 import "fmt"
 import "sys"