scanner: add support for @VMOD_FILE

doc/docs.md

@@ -1622,6 +1622,32 @@ $if debug {
 If you want an `if` to be evaluated at compile time it must be prefixed with a `$` sign. Right now it can only be used to detect
 an OS or a `-debug` compilation option.
 
+## Compile time pseudo variables
+
+V also gives your code access to a set of pseudo string variables, that are substituted at compile time:
+
+- `@FN` => replaced with the name of the current V function
+- `@MOD` => replaced with the name of the current V module
+- `@STRUCT` => replaced with the name of the current V struct
+- `@FILE` => replaced with the path of the V source file
+- `@LINE` => replaced with the V line number where it appears (as a string).
+- `@COLUMN` => replaced with the column where it appears (as a string).
+- `@VEXE` => replaced with the path to the V compiler
+- `@VHASH`  => replaced with the shortened commit hash of the V compiler (as a string).
+- `@VMOD_FILE` => replaced with the contents of the nearest v.mod file (as a string).
+
+That allows you to do the following example, useful while debugging/logging/tracing your code:
+```v
+eprintln( 'file: ' + @FILE + ' | line: ' + @LINE + ' | fn: ' + @MOD + '.' + @FN)
+```
+
+Another example, is if you want to embed the version/name from v.mod *inside* your executable:
+```v
+import v.vmod
+vm := vmod.decode( @VMOD_FILE ) or { panic(err) }
+eprintln('$vm.name $vm.version\n $vm.description')
+```
+
 ## Reflection via codegen
 
 Having built-in JSON support is nice, but V also allows you to create efficient