feat(:source, nvim_exec): defer script item creation until s:var access

For anonymous scripts, defer the creation of script items until an attempt to access a script-local
variable is made. This dramatically reduces the number of script items created when using lots of
vim.cmd and nvim_exec especially.

This will mean <SID> usage fails until a script-local variable access is first made.

test/functional/api/vim_spec.lua

@@ -89,6 +89,14 @@ describe('API', function()
 
     it(':verbose set {option}?', function()
       nvim('exec', 'set nowrap', false)
+      eq('nowrap\n\tLast set from anonymous :source',
+        nvim('exec', 'verbose set wrap?', true))
+
+      -- Using script var to force creation of a script item
+      nvim('exec', [[
+        let s:a = 1
+        set nowrap
+      ]], false)
       eq('nowrap\n\tLast set from anonymous :source (script id 1)',
         nvim('exec', 'verbose set wrap?', true))
     end)
@@ -155,6 +163,20 @@ describe('API', function()
           let s:pirate = 'script-scoped varrrrr'
           call nvim_exec('echo s:pirate', 1)
         ]], false))
+
+      -- Script items are created only on script var access
+      eq('1\n0', nvim('exec', [[
+          echo expand("<SID>")->empty()
+          let s:a = 123
+          echo expand("<SID>")->empty()
+        ]], true))
+
+      eq('1\n0', nvim('exec', [[
+          echo expand("<SID>")->empty()
+          function s:a() abort
+          endfunction
+          echo expand("<SID>")->empty()
+        ]], true))
     end)
 
     it('non-ASCII input', function()