Add a WAT test for interfacing JS via WASM

[certik]

Original issue: https://gitlab.com/lfortran/lfortran/-/issues/771

I would do the following:

$ lfortran --show-wat a.f90 
warning: Function with no body
  --> a.f90:73:9 - 76:23
   |
73 |            subroutine show_img(n, m, A) bind(c)
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
...
   |
76 |            end subroutine
   | ...^^^^^^^^^^^^^^^^^^^^^^^ show_img

warning: WASM: Calls to C subroutine are not yet supported
  --> a.f90:73:9 - 76:23
   |
73 |            subroutine show_img(n, m, A) bind(c)
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
...
   |
76 |            end subroutine
   | ...^^^^^^^^^^^^^^^^^^^^^^^ Function: calls show_img


Note: if any of the above error or warning messages are not clear or are lacking
context please report it to us (we consider that a bug that needs to be fixed).
(module
    (type (;0;) (func (param i32) (result)))
    (type (;1;) (func (param i64) (result)))
    (type (;2;) (func (param f32) (result)))
    (type (;3;) (func (param f64) (result)))
    (type (;4;) (func (param i32 i32) (result)))
    (type (;5;) (func (param) (result)))
    (type (;6;) (func (param i32) (result)))
    (import "js" "print_i32" (func (;0;) (type 0)))
    (import "js" "print_i64" (func (;1;) (type 1)))
    (import "js" "print_f32" (func (;2;) (type 2)))
    (import "js" "print_f64" (func (;3;) (type 3)))
    (import "js" "print_str" (func (;4;) (type 4)))
    (import "js" "flush_buf" (func (;5;) (type 5)))
    (import "js" "set_exit_code" (func (;6;) (type 6)))
    (import "js" "memory" (memory (;0;) 100 100))
)

where a.f90 is the code from the current dev.lfortran.org:

program mandelbrot
    implicit none
    integer  , parameter :: rk       = 8
    integer  , parameter :: i_max    =  600
    integer  , parameter :: j_max    =  450
    integer  , parameter :: n_max    =  100
    real (rk), parameter :: x_centre = -0.5_rk
    real (rk), parameter :: y_centre =  0.0_rk
    real (rk), parameter :: width    =  4.0_rk
    real (rk), parameter :: height   =  3.0_rk
    real (rk), parameter :: dx_di    =   width / i_max
    real (rk), parameter :: dy_dj    = -height / j_max
    real (rk), parameter :: x_offset = x_centre - 0.5_rk * (i_max + 1) * dx_di
    real (rk), parameter :: y_offset = y_centre - 0.5_rk * (j_max + 1) * dy_dj
    integer :: image(i_max, j_max)
    integer :: image_t(j_max, i_max)
    integer   :: i
    integer   :: j
    integer   :: n
    real (rk) :: x
    real (rk) :: y
    real (rk) :: x_0
    real (rk) :: y_0
    real (rk) :: x_sqr
    real (rk) :: y_sqr

    do j = 1, j_max
    y_0 = y_offset + dy_dj * j
    do i = 1, i_max
        x_0 = x_offset + dx_di * i
        x = 0.0_rk
        y = 0.0_rk
        n = 0
        do
        x_sqr = x ** 2
        y_sqr = y ** 2
        if (x_sqr + y_sqr > 4.0_rk) then
            image(i,j) = 255
            exit
        end if
        if (n == n_max) then
            image(i,j) = 0
            exit
        end if
        y = y_0 + 2.0_rk * x * y
        x = x_0 + x_sqr - y_sqr
        n = n + 1
        end do
    end do
    end do

    ! print '(a)', 'P2'
    ! print '(i0, 1x, i0)', i_max, j_max
    ! print '(i0)', 255
    ! do j = 1, 3
    !   do i = 1, 3
    !     print '(i0)', image(i,j)
    !   end do
    ! end do

    do i = 1, i_max
        do j = 1, j_max
            image_t(j, i) = image(i, j)
        end do
    end do

    print *, "The result is as follows:"
    call show_img(j_max, i_max, image_t)

    print *, "Thank you! Hope you had fun!"

    interface
        subroutine show_img(n, m, A) bind(c)
        integer, intent(in) :: n, m
        integer, intent(in) :: A(n,m)
        end subroutine
    end interface
end program mandelbrot