feat(:restart): v:startreason #40186

Problem:
It's clumsy for scripts to handle a "restart", without custom mappings or
global vars.

Solution:
Introduce `v:startreason`

(cherry picked from commit ae426ee465)

test/functional/vimscript/vvars_spec.lua

@@ -48,3 +48,11 @@ describe('v:argf', function()
     eq({ abs1, abs2, abs3 }, n.eval('v:argf'))
   end)
 end)
+
+describe('v:startreason', function()
+  it('is read-only and starts as "normal"', function()
+    n.clear { args = { '--cmd', 'let g:early_startreason = v:startreason' } }
+    eq('normal', eval('g:early_startreason'))
+    t.matches('E46', t.pcall_err(command, "let v:startreason = 'restart'"))
+  end)
+end)