fix command literals with trailing backslashes

[simeonschaub]

`\\` currently throws an error because it is parsed as @cmd "\\"
due to escaping in the parser and shell_parse therefore thinks it's a
dangling backslash.

[simeonschaub]

Bumping this again.

[vtjnash]

I tried finishing this PR, but Pkg currently relies on the existing behavior in one place:

   8 function gen_build_code(build_file::String; inherit_project::Bool = false)
   7     code = """                                    
   6         $(Base.load_path_setup_code(false))                     
   5         cd($(repr(dirname(build_file))))
   4         include($(repr(build_file)))                     
   3         """                            
   2     return ```
   1         $(Base.julia_cmd()) -O0 --color=no --history-file=no
1032         $(inherit_project ? `--project=$(Base.active_project())` : ``)
   1         --eval $code
   2         ```
   3 end                                                            

The trouble being that macro cmd can't precisely reverse the transform (as this PR desires to do), since the user might have called it with ``` syntax, which allowed the user to embed ` without escaping them, which otherwise (with single ` syntax) is a reversible transform

[simeonschaub]

Superseded by #52001