Lua output fixes (#1811)

* encoder_lua: Handle explicitly positive infinity

* encoder_lua: Fix inclusion of pre-/suffix when prettyPrinted

It seems certain operations like --prettyPrint or subset selections does
not produce a DocumentNode, which is where the lua pre- and suffix was
printed, causing those to be omitted.

* encoder_lua: Improve Tag handling robustness

Using the method call seems more reliable in case the input parser
forgets to set the tag.

pkg/yqlib/lua_test.go

@@ -135,6 +135,8 @@ numbers:
   - octal: 0o30
   - float: 123.45
   - infinity: .inf
+    plus_infinity: +.inf
+    minus_infinity: -.inf
   - not: .nan
 `,
 		expected: `return {
@@ -161,6 +163,8 @@ numbers:
 		},
 		{
 			["infinity"] = (1/0);
+			["plus_infinity"] = (1/0);
+			["minus_infinity"] = (-1/0);
 		},
 		{
 			["not"] = (0/0);