Fix line continuations in literals

[Morriar]

Motivation

Port of whitequark/parser#539.

This PR introduces a few changes in how literal with the \\\n continuation are parsed.

Heredoc changes

All variations of the non-plain heredoc like HERE, "HERE", `HERE`

<<-HERE
a\
b
HERE

Are now correctly parsed as s(:str, "ab\n") instead of s(:str, "a\nb\n").

This apply also for the squiggly heredoc non-plain variations:

<<~HERE
  a\
  b
HERE

That is now parsed as s(:str, "a b\n") instead of s(:str, "a\n b\n") (note that we still have a bug in the dedenting of the heredoc but this can be fixed independently from this PR).

Plain 'HERE" is unchanged.

Literal strings changes

Lines continuations are correctly parsed for interpolated strings, " ", %{ } and %Q{ }:

a\
b

Now returns s(:str, "ab") instead of s(:str, "a\nb\n").

Plain strings with ' ' and %q{ } are unchanged.

Literal symbols changes

Lines continuations are correctly parsed for symbols:

:a\
b

Now returns s(:sym, :ab) instead of s(:sym, :a\nb).

Literal arrays of symbols (interpolated or not) are unchanged.

Literal shell commands changes

Lines continuations are correctly parsed for shell commands:

`a\
b`

Now returns s(:xstr, s(:str, "ab"))) instead of s(:xstr, s(:str, "a\nb"))).

Test plan

See included automated tests.

[jez]

We have a policy of testing changes to Sorbet against Stripe's codebase before
merging them. I've kicked off a test run for the current PR. When the build
finishes, I'll share with you whether or how it failed. Thanks!

Stripe employees can see the build result here:

→ https://go/builds/bui_HzznrNppbUr6A5