`$embed_file` should allow pseudo variables as argument

[Apocryphon-X]

V doctor:

OS: linux, Ubuntu 20.04.4 LTS (WSL 2)
Processor: 12 cpus, 64bit, little endian, 11th Gen Intel(R) Core(TM) i5-11400H @ 2.70GHz
CC version: cc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0

getwd: /home/apocryphon/tests/v
vmodules: /home/apocryphon/.vmodules
vroot: /home/apocryphon/packages/v
vexe: /home/apocryphon/packages/v/v
vexe mtime: 2022-07-24 17:00:00
is vroot writable: true
is vmodules writable: true
V full version: V 0.3.0 d6de533.297cb5f

Git version: git version 2.25.1
Git vroot status: weekly.2022.29-57-g297cb5f8
.git/config present: true
thirdparty/tcc status: thirdparty-linux-amd64 827f7452

What did you do?

self := $embed_file(@FILE)

What did you expect to see? No errors.

What did you see instead?

meta.v:18:22: error: unexpected token `@`, expecting string
   16 | 
   17 | fn main() {
   18 |     self := $embed_file(@FILE)
      |                         ~~~~~
   19 | }
   20 |