Implement prefix-based interactive commands via ! (issue #14)

Add !command syntax that runs commands with inherited terminal (no
capture). The lexer treats ! as a statement-starting token, reading
to end-of-line with the same interpolation/escape/pipe support as
backtick commands. The C function sets _ = {code=N, stdout="", stderr=""}.

testes/literals.lua

@@ -124,9 +124,12 @@ lexerror([['alo \98]], "<eof>")
 -- valid characters in variable names
 for i = 0, 255 do
   local s = string.char(i)
+  -- skip '!' which is now a valid statement starter (interactive commands)
+  if s == '!' then goto continue end
   assert(not string.find(s, "[a-zA-Z_]") == not load(s .. "=1", ""))
   assert(not string.find(s, "[a-zA-Z_0-9]") ==
          not load("a" .. s .. "1 = 1", ""))
+  ::continue::
 end