RFC: strip spaces after backslash-escaped newline

This was discussed on Slack with @StefanKarpinski and @BioTurboNick and
the general consensus was that stripping all spaces following a
backslash-escaped newline would make more sense than the current
behavior. One subtle distinction here is whether this stripping should
happen before or after triple-quoted strings. Right now it happens after
detenting so

```julia
"""
  a\
b"""
```

produces `"  ab"`, whereas if it would be the other way around, escaped
newlines would not affect detenting so the spaces preceding the `a`
would be removed.

base/shell.jl

@@ -91,6 +91,10 @@ function shell_parse(str::AbstractString, interpolate::Bool=true;
                 if !isempty(st) && peek(st)[2] == '\n'
                     i = consume_upto!(arg, s, i, j) + 1
                     _ = popfirst!(st)
+                    while !isempty(st) && peek(st)[2] in (' ', '\t')
+                        i = nextind(str, i)
+                        _ = popfirst!(st)
+                    end
                 elseif in_double_quotes
                     isempty(st) && error("unterminated double quote")
                     k, c′ = peek(st)

src/julia-parser.scm

@@ -2194,7 +2194,10 @@
             (cond ((eof-object? c))
                   (preceding-backslash?
                    (if (not (eqv? c #\newline))
-                       (begin (write-char #\\ out) (write-char c out)))
+                       (begin (write-char #\\ out) (write-char c out))
+                       ((define (loop-)
+                          (if (memv (peek-char in) '(#\space #\tab))
+                              (begin (take-char in) (loop-))))))
                    (loop #f))
                   ((eqv? c #\\) (loop #t))
                   (else (write-char c out) (loop #f)))))

test/syntax.jl

@@ -2837,7 +2837,7 @@ end
     @test "a\
 b" == "ab"
     @test "a\
-    b" == "a    b"
+    b" == "ab"
     @test raw"a\
 b" == "a\\\nb"
     @test "a$c\
@@ -2851,7 +2851,7 @@ b" == "acb"
           b""" == "ab"
     @test """
           a\
-            b""" == "a  b"
+            b""" == "ab"
     @test """
             a\
           b""" == "  ab"
@@ -2894,7 +2894,7 @@ b" == "acb"
     @test `a\
 b` == `ab`
     @test `a\
-    b` == `a    b`
+    b` == `ab`
     @test `a$c\
 b` == `acb`
     @test `"a\
@@ -2910,7 +2910,7 @@ b'` == `$("a\\\nb")`
           b``` == `ab`
     @test ```
           a\
-            b``` == `a  b`
+            b``` == `ab`
     @test ```
             a\
           b``` == `  ab`