evaluator: support line continuation with backslash-newline in multiline string literals

[rogpeppe]

Swift multiline strings support the use of a backslash at the end of a line to join that line to the following line.

CUE should probably support this too.

For example:

x: """
    abc\
    def
    """

would be equivalent to:

x: "abcdef"

[uhthomas]

Hmm, what if you really did want abc\\ndef? I guess abc\\? I'm not sure.

[myitcv]

@rogpeppe please can you link to the relevant part of the Swift spec/similar?

[myitcv]

FWIW the CUE spec already seems to suggest that CUE supports this:

https://tip.cuelang.org/docs/references/spec/#string-and-byte-sequence-literals

A backslash at the end of a line elides the line terminator that follows it. This may not escape the final newline inside a multiline string: that newline is already implicitly elided.

But I can't find a time since v0.2.2 when this was supported.

[myitcv]

Turns out that this simply didn't get implemented.