Inspired by a recent editor baked into the FORTH REPL in 30 lines of code, here’s one baked into the Lua REPL. Requires 100 lines, though. Something like this should be doable for BASIC as well.
-- line editor in the Lua REPL
-- available commands:
--  l'1' - moves cursor down by 1 line
--  l'-2' - moves cursor up by 2 lines
--  c'abc' - changes line at cursor to 'abc'
--  a'abc' - appends 'abc' below cursor line, moves cursor to it
--  s'' - shows current screen (up to N lines)
--  n'' - page down
--  b'' - page up
--  f'a' - find line matching 'a' downwards with wrap
--  F'a' - find line matching 'a' upwards with wrap

-- command/fn args are often strings so I can avoid pressing
-- the shift key
-- e.g. n() would work like n'', but require pressing shift.

Filename = ''
Lines = {}

function save(filename)
  if #filename == 0 and #Filename > 0 then
    filename = Filename
  end
  if #filename == 0 then
    print('file?')
    return
  end
  io.output(filename)
  for _,line in ipairs(Lines) do
    io.write(line..'\n')
  end
  io.close()
end

Cursor_line = 0  -- valid range: 0 + [1..#Lines]

function clamp()
  Cursor_line = math.max(Cursor_line, 1)
  Cursor_line = math.min(Cursor_line, #Lines)  -- might become 0 if no Lines
end

Screen_top = 1
N = 4  -- 32  -- number of lines to display per screen

function scroll()
  while Cursor_line > Screen_top+N-1 do
    Screen_top = Screen_top+N
  end
  while Cursor_line < Screen_top do
    Screen_top = Screen_top-N
  end
end

-- show current screen
function s(linestr)
  if #linestr > 0 then
    Screen_top = int(linestr)
  end
  local l = Screen_top
  print('====', Filename, l)
  for i = l, math.min(l+N-1,#Lines) do
    if i == Cursor_line then io.write('\027[7m') end
    if i == Cursor_line and #Lines[i] == 0 then
      print(' ')  -- indicate cursor on empty line
    else
      print(Lines[i])
    end
    if i == Cursor_line then io.write('\027[0m') end
  end
  print('----')
end

-- add new line below cursor and move cursor to it
function a(line)
  table.insert(Lines, Cursor_line+1, line)
  Cursor_line = Cursor_line+1
  clamp() scroll() s''
end

-- change line at cursor
function c(line)
  if Cursor_line == 0 then return end
  Lines[Cursor_line] = line
  Cursor_line = Cursor_line+1
  clamp() scroll() s''
end

-- move cursor line relatively
function l(nstr)
  local n = int(nstr)
  Cursor_line = Cursor_line+n
  clamp() scroll() s''
end

-- scroll
function n()
  if Screen_top + N <= #Lines then
    Screen_top = Screen_top + N
    Cursor_line = Screen_top
  end
  s''
end

function b()
  if Screen_top - N > 0 then
    Screen_top = Screen_top - N
    Cursor_line = Screen_top
  end
  s''
end

-- find
function f(pat)
  for i = Cursor_line+1, #Lines do
    if Lines[i]:find(pat) then
      Cursor_line = i
      clamp() scroll() s'' return
    end
  end
  for i = 1, Cursor_line-1 do
    if Lines[i]:find(pat) then
      Cursor_line = i
      clamp() scroll() s'' return
    end
  end
end
function F(pat)
  for i = Cursor_line-1, 1, -1 do
    if Lines[i]:find(pat) then
      Cursor_line = i
      clamp() scroll() s'' return
    end
  end
  for i = #Lines, Cursor_line+1, -1 do
    if Lines[i]:find(pat) then
      Cursor_line = i
      clamp() scroll() s'' return
    end
  end
end

function int(s) return math.floor(tonumber(s)) end

if #arg > 0 then
  Filename = arg[1]
  print('loading '..Filename)
  Lines = {}
  for line in io.lines(Filename) do
    table.insert(Lines, line)
  end
  print(#Lines, 'lines')
  Cursor_line = 1
end

s'1'