Expose lush global as a standard library (issue #26)

Register luaopen_lush() in stdlibs so the lush table is available as a
global. Users can now extend the shell by adding functions to
lush.builtins. The OP_LUSH VM access via integer keys is preserved.

testes/lush/lushlib.lua

@@ -0,0 +1,111 @@
+-- testes/lush/lushlib.lua
+-- Tests for the lush standard library (issue #26).
+
+print "testing lush library"
+
+-- === lush global exists and is a table ===
+
+do
+  assert(type(lush) == "table", "lush should be a table")
+end
+
+
+-- === named functions are accessible ===
+
+do
+  assert(type(lush.command) == "function", "lush.command missing")
+  assert(type(lush.interactive) == "function", "lush.interactive missing")
+  assert(type(lush.getenv) == "function", "lush.getenv missing")
+  assert(type(lush.setenv) == "function", "lush.setenv missing")
+  assert(type(lush.subcmd) == "function", "lush.subcmd missing")
+end
+
+
+-- === lush.command works like backtick ===
+
+do
+  local r = lush.command("echo hello")
+  assert(r.code == 0, "lush.command failed")
+  assert(r.stdout == "hello\n", "expected 'hello\\n', got: " .. r.stdout)
+end
+
+
+-- === lush.getenv / lush.setenv ===
+
+do
+  lush.setenv("_LUSH_TEST_VAR", "42")
+  assert(lush.getenv("_LUSH_TEST_VAR") == "42")
+  lush.setenv("_LUSH_TEST_VAR", nil)
+  assert(lush.getenv("_LUSH_TEST_VAR") == nil)
+end
+
+
+-- === lush.builtins exists and has cd ===
+
+do
+  assert(type(lush.builtins) == "table", "lush.builtins missing")
+  assert(type(lush.builtins.cd) == "function", "lush.builtins.cd missing")
+  assert(type(lush.builtins.exec) == "function", "lush.builtins.exec missing")
+  assert(type(lush.builtins.umask) == "function", "lush.builtins.umask missing")
+end
+
+
+-- === user-defined builtin via backtick ===
+
+do
+  lush.builtins.greet = function(cmd, name)
+    -- builtins return {code, stdout, stderr}
+    local t = {}
+    t.code = 0
+    t.stdout = "hello " .. (name or "world") .. "\n"
+    t.stderr = ""
+    return t
+  end
+
+  local r = `greet lush`
+  assert(r.code == 0, "user builtin failed")
+  assert(r.stdout == "hello lush\n",
+         "expected 'hello lush\\n', got: " .. r.stdout)
+
+  -- clean up
+  lush.builtins.greet = nil
+end
+
+
+-- === user-defined builtin via lush.command ===
+
+do
+  lush.builtins.myecho = function(cmd, ...)
+    local args = {...}
+    local t = {}
+    t.code = 0
+    t.stdout = table.concat(args, " ") .. "\n"
+    t.stderr = ""
+    return t
+  end
+
+  local r = lush.command("myecho foo bar")
+  assert(r.code == 0)
+  assert(r.stdout == "foo bar\n",
+         "expected 'foo bar\\n', got: " .. r.stdout)
+
+  lush.builtins.myecho = nil
+end
+
+
+-- === OP_LUSH still works (backtick, $VAR, !cmd syntax) ===
+
+do
+  -- backtick
+  local r = `echo works`
+  assert(r.stdout == "works\n")
+
+  -- $VAR expansion
+  lush.setenv("_LUSH_LIB_TEST", "yes")
+  local val = $_LUSH_LIB_TEST
+  assert(val == "yes", "expected 'yes', got: " .. tostring(val))
+  lush.setenv("_LUSH_LIB_TEST", nil)
+end
+
+
+print "OK"