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.

src/nvim/ex_cmds2.h

@@ -16,6 +16,31 @@
 #define CCGD_ALLBUF     8       // may write all buffers
 #define CCGD_EXCMD      16      // may suggest using !
 
+/// Also store the dev/ino, so that we don't have to stat() each
+/// script when going through the list.
+typedef struct scriptitem_S {
+  char_u *sn_name;
+  bool file_id_valid;
+  FileID file_id;
+  bool sn_prof_on;              ///< true when script is/was profiled
+  bool sn_pr_force;             ///< forceit: profile functions in this script
+  proftime_T sn_pr_child;       ///< time set when going into first child
+  int sn_pr_nest;               ///< nesting for sn_pr_child
+  // profiling the script as a whole
+  int sn_pr_count;              ///< nr of times sourced
+  proftime_T sn_pr_total;       ///< time spent in script + children
+  proftime_T sn_pr_self;        ///< time spent in script itself
+  proftime_T sn_pr_start;       ///< time at script start
+  proftime_T sn_pr_children;    ///< time in children after script start
+  // profiling the script per line
+  garray_T sn_prl_ga;           ///< things stored for every line
+  proftime_T sn_prl_start;      ///< start time for current line
+  proftime_T sn_prl_children;   ///< time spent in children for this line
+  proftime_T sn_prl_wait;       ///< wait start time for current line
+  linenr_T sn_prl_idx;          ///< index of line being timed; -1 if none
+  int sn_prl_execed;            ///< line being timed was executed
+} scriptitem_T;
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "ex_cmds2.h.generated.h"
 #endif