Increased TInstr field sizes: allow long jumps and 65535 VM registers (#12777)

* Increased regBx size from 16 to 24 bits to increase jump range in the VM
from 32K to 8M instructions. Fixes #12727

* Increased VM TInst register field sizes to 16 bits to allow up to 65535 VM registers per proc

* Added test case for >255 VM registers

tests/vm/tfarjump.nim

@@ -0,0 +1,14 @@
+# Test a VM relative jump with an offset larger then 32767 instructions.
+
+import macros
+
+static:
+  var a = 0
+  macro foo(): untyped =
+    let s = newStmtList()
+    for i in 1..6554:
+      s.add nnkCommand.newTree(ident("inc"), ident("a"))
+    quote do:
+      if true:
+        `s`
+  foo()