Add file_exist function to std

[clysto]

How to write a test about file operations? I saw that the test does not include a test for file_read/file_write. Can you give me an example?

[b1ek]

How to write a test about file operations? I saw that the test does not include a test for file_read/file_write. Can you give me an example?

that is weird that there are no tests for those functions.

tests are written like this:

#[test]
fn name_of_the_test() {
    test_amber!("// put amber code here", "what the code should output");
}

i guess that if the other file functions are not covered for now, we could sweep that issue under the carpet in the scope of this PR. (see #117)

[Ph0enixKM]

I think that we will focus on std more in the next milestone. Right now we have to focus on making Amber usable in terms of making generated bash more user friendly and more debuggable I think. Although if you're adding file_exist function, please add dir_exist function as well @clysto. Thanks! The tests regarding STD will be added as soon as we improve the DX