CS1 tagged template literals (and CS2 interpolated strings as template literals) (#4352)

* Add initial support for template literals with no
interpolation

* Change ‘unexpected string’ error message tests to
use number not identifier prefix.

Identifer prefixes are now valid as tagged
template literals

* Test tagged template literals for non-interpolated
strings and tag function.

* Tagged template literals work for pure Strings.

Pull tagged template definition up to Invocation
level in grammar, enabling chained invocation calls.

We can view a tagged template is a special form
of function call.

* Readying for StringWithInterpolations work.

* Tweaks.

* Fix style

* Pass StringWithInterpolations parameter straight
into Call constructor.

StringWithInterpolations will be output as
template literal, so already in correct form for
outputting tagged template literal.

* Strip down compileNode for StringWithInterpolations

* Done StringLiteral case for interpolated Strings

* Remove need for TemplateLiteral

* Simplify code.

* Small code tidy

* Interpolated strings now outputting as template literals.

Still needs comprehensive testing.

* Move error message tests into error_messages.coffee; remove test that is testing for a Node runtime error

* Split up tests that were testing multiple things per test, so that each test tests only one thing

* Edge cases: tagged template literals containing interpolated strings or even internal tagged template literals

* Make more concise, more idiomatic style

* Pull back extreme indentation

* Restore and fix commented-out tests

* Edge case: tagged template literal with empty string

* Only use new ES2015 interpolated string syntax if we’re inside a tagged template literal; this keeps this PR safe to merge into CoffeeScript 1.x. Remove the code from this commit to make all interpolated strings use ES2015 syntax, for CoffeeScript 2.

* Compiler now _doesn’t_ use template literals.

* Expand tagged template literal tests

* Move ‘Unexpected string’ error message tests into
tagged template literal section.

‘Unexpected string’ is not reported in these test
scenarios anymore. Instead, we error that the
prefixing literal is not a function.

* Don’t unwrap StringWithInterpolations.

Saw bug with program consisting of “#{2}” not
compiling with template literals. Root cause was
that Block.compileNode was unwrapping interpolated
string and so didn’t use compileNode logic at
StringWithInterpolations level.

* No need to bracket interpolated strings any more.

When interpolated string looks like `hello ${2}`,
no extract brackets are needed, as the `s mark the
beginning and end.

* Show html templating with tagged template literals

* Multiline should match multiline

* Comment out unnecessary `unwrap`, which is only needed for CoffeeScript 2 all-ES2015 syntax output

Author: greghuc