Add `Before` for column selection

[mathieu17g]

Wouldn't it be great to add the Before selector to Not, Between, Cols, All and regex column selectors?

This would simplify a bit the use case where you add a new column with transform and move it to a specific position, for instance around the column used for the transformation.

Example:

julia> df = DataFrame(:a => [1, 2, 3, 4], :b1 => [2, 3, 4, 1], :B2 => [3, 4, 1, 2], :c => [4, 1, 2, 3])       
       transform!(df, [:b1, :B2] => ByRow(+) => :b)
       select!(df, Cols(1:findfirst(==("b1"), names(df))-1, :b, :))
4×5 DataFrame
 Row │ a      b      b1     B2     c     
     │ Int64  Int64  Int64  Int64  Int64
─────┼───────────────────────────────────
   1 │     1      5      2      3      4
   2 │     2      7      3      4      1
   3 │     3      5      4      1      2
   4 │     4      3      1      2      3

julia> 

The reordering would be

julia> df = DataFrame(:a => [1, 2, 3, 4], :b1 => [2, 3, 4, 1], :B2 => [3, 4, 1, 2], :c => [4, 1, 2, 3])       
       transform!(df, [:b1, :B2] => ByRow(+) => :b)
       select!(df, Cols(Before(:b1), :b, :))
4×5 DataFrame
 Row │ a      b      b1     B2     c     
     │ Int64  Int64  Int64  Int64  Int64
─────┼───────────────────────────────────
   1 │     1      5      2      3      4
   2 │     2      7      3      4      1
   3 │     3      5      4      1      2
   4 │     4      3      1      2      3

After could also be useful

julia> df = DataFrame(:a => [1, 2, 3, 4], :b1 => [2, 3, 4, 1], :B2 => [3, 4, 1, 2], :c => [4, 1, 2, 3])       
       transform!(df, [:b1, :B2] => ByRow(+) => :b)
       select!(df, Cols(1:findfirst(==("B2"), names(df)), :b, :))
4×5 DataFrame
 Row │ a      b1     B2     b      c     
     │ Int64  Int64  Int64  Int64  Int64
─────┼───────────────────────────────────
   1 │     1      2      3      5      4
   2 │     2      3      4      7      1
   3 │     3      4      1      5      2
   4 │     4      1      2      3      3

Would become

julia> df = DataFrame(:a => [1, 2, 3, 4], :b1 => [2, 3, 4, 1], :B2 => [3, 4, 1, 2], :c => [4, 1, 2, 3])       
       transform!(df, [:b1, :B2] => ByRow(+) => :b)
       select!(df, Cols(Not(After(:B2)), :b, :))
4×5 DataFrame
 Row │ a      b1     B2     b      c     
     │ Int64  Int64  Int64  Int64  Int64
─────┼───────────────────────────────────
   1 │     1      2      3      5      4
   2 │     2      3      4      7      1
   3 │     3      4      1      5      2
   4 │     4      1      2      3      3