Make assets:install smarter with symlinks

| Q             | A
| ------------- | ---
| Bug fix?   | no
| New feature?      | yes
| BC breaks?      | no
| Deprecations?      | no
| Tests pass?      | -
| Fixed tickets     | #11297
| License     | MIT
| Doc PR | -
There is a `function_exists` call before and one in `Filesystem::symlink`. I see that as of 5.3.0, `symlink` looks like it exists for Windows too. Should those checks be removed?