refactor!: delete insertmode (#18547)

Neovim already removed `evim` (or any similar flags). The 'insertmode'
option is a weird remnant, so get rid of it.

The 'insertmode' option is replaced with a script that closely emulates
the option. This script is documented at :help 'insertmode'

test/functional/legacy/edit_spec.lua

@@ -1,25 +0,0 @@
--- Test for edit functions
--- See also: src/nvim/testdir/test_edit.vim
-
-local helpers = require('test.functional.helpers')(after_each)
-local source = helpers.source
-local eq, eval = helpers.eq, helpers.eval
-local funcs = helpers.funcs
-local clear = helpers.clear
-
-describe('edit', function()
-  before_each(clear)
-
-  it('reset insertmode from i_ctrl-r_=', function()
-    source([=[
-      call setline(1, ['abc'])
-      call cursor(1, 4)
-      call feedkeys(":set im\<cr>ZZZ\<c-r>=setbufvar(1,'&im', 0)\<cr>",'tnix')
-    ]=])
-    eq({'abZZZc'}, funcs.getline(1,'$'))
-    eq({0, 1, 1, 0}, funcs.getpos('.'))
-    eq(0, eval('&im'))
-  end)
-
-end)
-