Make fileExists and dirExists in os module available at compile time even when --os:any

[demotomohiro]

This PR partly fix #19414.
With this PR, fileExists and dirExists works at compile time by importing std/private/oscommon even when --os:any option is set.

For example,
testosmod.nim:

import std/private/oscommon

static:
  doAssert fileExists(currentSourcePath())
  doAssert dirExists("..")

nim c --os:any -d:useMalloc testosmod.nim

By changing code in the same way as this PR, you will be able to import os module and procs and iterators in os module that can be used at compile time become available even when --os:any.
But changing code in this way is OK?

If there is fileExists or dirExists proc that is called at runtime when --os:any or target is unsupported OS, it should be a compile error.
But how to do so?

I defined following supportCompileTime pragma and added it to fileExists and dirExists procs so that they have compileTime pragma on unsupported OS.
But calling them at running doesn't cause compile error.

when supportedOS:
  {.pragma: supportCompileTime.}
else:
  # On unsupported OS, you can call the proc at compile but at runtime.
  {.pragma: supportCompileTime, compileTime.}

And cannot forbid using walkDir iterator at runtime with compileTime pragma because adding compileTime pragama to iterators is a compile error.

[Araq]

What about new procs staticFileExists and staticDirExists that are available at compile-time, always?

[demotomohiro]

I'm ok with that idea.
Then, staticFileExists and staticDirExists should be defined in oscommon module like fileExists or dirExists?
Or create a new module like 'staticos' and define them in that module?

Currently, fileExists, dirExists and walkDir can be used at compile time unless you do cross compiling.
People might write large code that uses these procs/iterator at compile-time.
Then, that code is compiled with --os:any and get compile error and need to replace all fileExists with staticFileExists.