all: add compile_error and compile_warn comptime functions

[StunxFS]

This PR adds the compile_error and compile_warn comptime functions.

Both are very useful for handling cases where, for example, a certain generic type, or an operating system, is not supported.

$ cat x.v
module main

$if linux {
    $compile_error('Linux is not supported')
}

fn main() {
}

$ ./v2 run x.v
x.v:4:5: error: Linux is not supported
    2 |
    3 | $if linux {
    4 |     $compile_error('Linux is not supported')
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    5 | }
    6 |

[larpon]

Nice work, I like the idea of being able to give compile time errors - what are compile warnings good for, though?

It reminds me a bit of the C/C++ -Wall-type of flags needed to activate or hide warnings when using C-based compilers. Either code compiles else the programmer is doing something wrong that deserves an error - what cases or examples do we have for giving a warning at compile time?

Maybe userland compile warnings should at least result in compile errors with -prod - like they do now in the internal system?

[medvednikov]

@StunxFS good job!

Can you add a test with expected output, I'll merge it then.

[spytheman]

It can be in vlib/v/checker/tests/ - a pair of .vv and .out files, where the .vv file has the example, and the .out file is produced by v vlib/v/checker/tests/compile_error.vv > vlib/v/checker/tests/compile_error.out .