Update for zig 0.15.1

[djmattyg007]

I'm getting an error when trying to include this as a dependency in my project that's using zig 0.15.2:

❯ zig build
/path/to/zigcache/p/termsize-0.1.0-AAAAAPwoAADDf7A2UP2G3_GRNBFqK9fo4gcxGTqjDxF2/build.zig:30:10: error: no field named 'root_source_file' in struct 'Build.TestOptions'
        .root_source_file = b.path("src/main.zig"),
         ^~~~~~~~~~~~~~~~
/usr/lib/zig/std/Build.zig:856:25: note: struct declared here
pub const TestOptions = struct {
                        ^~~~~~
referenced by:
    runBuild__anon_76429: /usr/lib/zig/std/Build.zig:2215:44
    dependencyInner__anon_74177: /usr/lib/zig/std/Build.zig:2195:29
    9 reference(s) hidden; use '-freference-trace=11' to see all references

Given the readme also references std.io.getStdOut() (which no longer exists in zig 0.15.1) it looks like this package generally needs updating to support the latest version of zig.

[djmattyg007]

I've opened #8 to resolve this.