feat(startup): provide v:argf for file arguments #35889

Problem:
- `:args` and `argv()` can change after startup.
- `v:arg` includes options/commands, not just files.
- Plugins (e.g. Oil) may rewrite directory args.

Solution:
- New read-only var `v:argf`: snapshot of file/dir args at startup.
- Unaffected by `:args` or plugins.
- Unlike `v:argv`, excludes options/commands.
- Paths are resolved to absolute paths when possible

Example:

    nvim file1.txt dir1 file2.txt
    :echo v:argf
    " ['/home/user/project/file1.txt', '/home/user/project/dir1', '/home/user/project/file2.txt']

runtime/lua/vim/_meta/vvars.lua

@@ -6,6 +6,21 @@ error('Cannot require a meta file')
 --- @class vim.v
 vim.v = ...
 
+--- The list of file arguments passed on the command line at startup.
+---
+--- Each filename is expanded to an absolute path, so that v:argf
+--- remains valid even if the current working directory changes later.
+---
+--- Unlike `v:argv`, this does not include option arguments
+--- such as `-u`, `--cmd`, or `+cmd`. Unlike `argv()`, it is not
+--- affected by later `:args`, `:argadd`, or plugin modifications.
+--- It also handles the `--` separator correctly, including only
+--- files specified after it.
+---
+--- This is a read-only snapshot of the original startup file arguments.
+--- @type string[]
+vim.v.argf = ...
+
 --- The command line arguments Vim was invoked with.  This is a
 --- list of strings.  The first item is the Vim command.
 --- See `v:progpath` for the command with full path.