std.Build.Step.ConfigHeader: Add custom style

[playday3008]

TLDR

Simply replaces names with values.

Example

• Somewhere in target C Header:

  /// @file: magic.h.in
  #define MAGIC_VERSION    X.YY
  
  int magic_version() {
      return MAGIC_VERSION;
  }

• Somewhere in build.zig

  const magic = b.addConfigHeader(
      .{
          .style = .{
              .custom = upstream.path("src/magic.h.in"),
          },
      },
      .{
          .@"X.YY" = 123456,
      },
  );

• Resulting C Header:

  /// @file: magic.h
  #define MAGIC_VERSION    123456
  
  int magic_version() {
      return MAGIC_VERSION;
  }

Real world use case

https://github.com/file/file/blob/7ed3febfcd616804a2ec6495b3e5f9ccb6fc5f8f/src/magic.h.in#L120
https://github.com/file/file/blob/7ed3febfcd616804a2ec6495b3e5f9ccb6fc5f8f/src/Makefile.am#L30

[playday3008]

The only concern I have is when someone would do something like this:

.{
    .ABCD = 1234, 
    .@"1234" = 5678,
}

This will basically replace ABCD with 5678, but if order is changed this will not occur, maybe it's a bug, but I'd say it's a feature.

[playday3008]

Also I'm not sure if my implementation is optimal enough, feel free to correct me or review the code