This PR fixes an AST and token incompatibility between Parser gem and `Prism::Translation::Parser`
for empty xstring literal.

## Parser gem (Expected)

```console
$ bundle exec ruby -Ilib -rparser/ruby33 -ve \
'buf = Parser::Source::Buffer.new("/tmp/s.rb"); buf.source = "``"; p Parser::Ruby33.new.tokenize(buf)'
ruby 3.3.0 (2023-12-25 revision https://github.com/ruby/prism/commit/5124f9ac75) [x86_64-darwin22]
[s(:xstr), [], [[:tXSTRING_BEG, ["`", #<Parser::Source::Range /tmp/s.rb 0...1>]],
[:tSTRING_END, ["`", #<Parser::Source::Range /tmp/s.rb 1...2>]]]]
```

## `Prism::Translation::Parser` (Actual)

Previously, the AST and tokens returned by the Parser gem were different:

```console
$ bunele exec ruby -Ilib -rprism -rprism/translation/parser33 -ve \
'buf = Parser::Source::Buffer.new("/tmp/s.rb"); buf.source = "``"; p Prism::Translation::Parser33.new.tokenize(buf)'
ruby 3.3.0 (2023-12-25 revision https://github.com/ruby/prism/commit/5124f9ac75) [x86_64-darwin22]
[s(:xstr, s(:str, "")), [], [[:tBACK_REF2, ["`", #<Parser::Source::Range /tmp/s.rb 0...1>]],
[:tSTRING_END, ["`", #<Parser::Source::Range /tmp/s.rb 1...2>]]]]
```

After this correction, the AST and tokens returned by the Parser gem are the same:

```console
$ bundle exec ruby -Ilib -rprism -rprism/translation/parser33 -ve \
'buf = Parser::Source::Buffer.new("/tmp/s.rb"); buf.source = "``"; p Prism::Translation::Parser33.new.tokenize(buf)'
ruby 3.3.0 (2023-12-25 revision https://github.com/ruby/prism/commit/5124f9ac75) [x86_64-darwin22]
[s(:xstr), [], [[:tXSTRING_BEG, ["`", #<Parser::Source::Range /tmp/s.rb 0...1>]],
[:tSTRING_END, ["`", #<Parser::Source::Range /tmp/s.rb 1...2>]]]]
```

https://github.com/ruby/prism/commit/4ac89dcbb5

Fixes ruby/prism#2480.

This PR fixes an AST incompatibility between Parser gem and `Prism::Translation::Parser`
for xstring literal with line breaks.

The following case in ruby/prism#2480 has already been addressed in ruby/prism#2576:

```ruby
"foo
bar"
```

https://github.com/ruby/prism/commit/cf85e72c55

In practice, the `BACKTICK` is mapped either as `:tXSTRING_BEG` or `:tBACK_REF2`.
The former is used in xstrings like `` `foo` ``, while the latter is utilized as
a back reference in contexts like `` A::` ``.
This PR will make corrections to differentiate the use of `BACKTICK`.

This mistake was discovered through the investigation of xstring.txt file.
The PR will run tests from xstring.txt file except for `` `f\oo` ``, which will still fail,
hence it will be separated into xstring_with_backslash.txt file.
This separation will facilitate addressing the correction at a different time.

https://github.com/ruby/prism/commit/49ad8df40a