Implement $(cmd) subcommand syntax in commands (issue #27)

Add $() for inline subcommand substitution that runs a command and
inserts its stdout (trailing newlines stripped) into the outer command
string. Supports nesting, and mixing with ${expr} and $NAME.

testes/lush/interpolation.lua

@@ -109,4 +109,54 @@ do
   assert(r.stdout == "$HOME\n", "got: " .. r.stdout)
 end
 
+-- $(cmd) subcommand: basic
+do
+  local r = `echo $(echo hello)`
+  assert(r.stdout == "hello\n", "got: " .. r.stdout)
+end
+
+-- $(cmd) strips trailing newline from subcommand output
+do
+  local r = `echo -$(echo world)-`
+  assert(r.stdout == "-world-\n", "got: " .. r.stdout)
+end
+
+-- $(cmd) multiple subcommands
+do
+  local r = `echo $(echo aaa) $(echo bbb)`
+  assert(r.stdout == "aaa bbb\n", "got: " .. r.stdout)
+end
+
+-- $(cmd) nested subcommand
+do
+  local r = `echo $(echo $(echo deep))`
+  assert(r.stdout == "deep\n", "got: " .. r.stdout)
+end
+
+-- $(cmd) with ${expr} inside subcommand
+do
+  local r = `echo $(echo ${1+1})`
+  assert(r.stdout == "2\n", "got: " .. r.stdout)
+end
+
+-- $(cmd) with $NAME inside subcommand
+do
+  local r = `echo $(echo $USER)`
+  assert(r.stdout == $USER .. "\n", "got: " .. r.stdout)
+end
+
+-- $(cmd) mixed with $NAME and ${expr}
+do
+  local x = 42
+  local r = `echo $(echo sub) $USER ${x}`
+  local expected = "sub " .. $USER .. " 42\n"
+  assert(r.stdout == expected, "got: " .. r.stdout)
+end
+
+-- escaped \$(cmd) remains literal
+do
+  local r = `echo \$(pwd)`
+  assert(r.stdout == "$(pwd)\n", "got: " .. r.stdout)
+end
+
 print "OK"