Implement basic Lua output support

[Zash]

First attempt at #1700

Supports basic serialization to Lua tables:

$ ./yq -n  -olua '.hello="\"world\"",.foo=["bar","baz",3,1.3]' 
return {["hello"]="\"world\"";["foo"]={"bar","baz",3,1.3,};};

No indentation or pretty printing yet, likely next steps.

Feedback appreciated, hence the Draft PR. (Haven't written Go in a couple of years so be gentle.)

[mikefarah]

Nice, looks great so far! You'll need to add some unit tests (which also generate docs) - see toml_test.go or xml_test.go for reference.

[Zash]

Okay, I think it does all I want it to now.

• Indentation
• Tests (uncertain of coverage)
• Documentation (generated)
• Optional unquoted string keys
• Optional global mode
• Line comments (but not head or foot comments)

yq$ ./yq -n -o=lua '.hello=["world","you"]'
return {
	["hello"] = {
		"world",
		"you",
	};
};
yq$ ./yq -n -o=lua --lua-unquoted '.hello=["world","you"]'
return {
	hello = {
		"world",
		"you",
	};
};
yq$ ./yq -n -o=lua --lua-globals '.hello=["world","you"]'
hello = {
	"world",
	"you",
};

[mikefarah]

Wow great work!