feat(update): add update by environment variable

[nkxxll]

Added the second option to update a snapshot test with an environment variable very inspired by the original snapshot testing library from TigerBeetle (see tigerbeetle/tigerbeetle/blob/main/src/testing/snaptest.zig#L109-L112).

This option was, tested but a unit test is kind of useless in this case. The test included the update of the CustomStruct structure like shown in the code snippet below (first snippet before update, second commands, third after update):

test "update env var" {
    const oh = OhSnap{};
    const foobar = CustomStruct{ .foo = 42, .bar = 23 };
    try oh.snap(
        @src(),
        \\
        ,
    ).expectEqual(foobar);
}

# fail on test "update env var"
zig build test
# command to update
SNAP_UPDATE=1 zig build test
# "update env var" test passes
zig build test

test "update env var" {
    const oh = OhSnap{};
    const foobar = CustomStruct{ .foo = 42, .bar = 23 };
    try oh.snap(
        @src(),
        \\ohsnap.CustomStruct
        \\  .foo: u64 = 42
        \\  .bar: u66 = 23
        ,
    ).expectEqual(foobar);
}

PS: I know the code is a little bit repetitive if you have an idea how make it look nicer I'm open for changes.

PPS: I know this is out of the blue. I looked at snapshot testing, then the blog entry from TigerBeetle, then thought I could outsource the snap lib from, them then saw your project and missed this feature while using it hope you like it. Like the idea of this project very much ❤️!