Fix indentation of xstring literal

[tompng]

Fixes wrong indent calculation of this input

irb(main):001* if false
irb(main):002> p `ls`
irb(main):003> end

Description

We need to distinguish Kernel.` from `xstring_literal`.

code	event	state	xstring_literal
`ls`	:on_backtick	EXPR_BEG	yes
class `ls`::A;end	:on_backtick	EXPR_CLASS	yes
p `ls`	:on_backtick	EXPR_CMDARG	yes
p x: `ls`	:on_backtick	EXPR_ARG|EXPR_LABELED	yes
p(`ls`)	:on_backtick	EXPR_BEG|EXPR_LABEL	yes
Kernel.`	:on_backtick	EXPR_ARG	no

The simplest way to check is using state != EXPR_ARG.

Known issues

As shown in the table below, checking by state != EXPR_ARG is not perfect. But there is no easy way.

code	event	state	xstring_literal
p p `ls`	:on_backtick	EXPR_ARG	yes
Kernel.`	:on_backtick	EXPR_ARG	no

# still fails to indent this
if false
p p `ls`
end