flags: support flags that are just combinations of other flags, not taking a new bit.

[programmerjake]

This came up in WebAssembly/wasi-gfx#71 (comment)

where WebGpu has a C definition:
https://github.com/webgpu-native/webgpu-headers/blob/bf8ddb91dc38ea11ec1b727dae1fa965c4207d22/webgpu.h#L1334

/**
 * `Red | Green | Blue | Alpha`.
 */
static const WGPUColorWriteMask WGPUColorWriteMask_All = 0x000000000000000F;

the generated WIT in WebAssembly/wasi-gfx#71 as of writing is:

flags gpu-color-write {
    red,
    green,
    blue,
    alpha,
    all,
}

which is incorrect in that all shouldn't be a new flag, just a shorthand for a combination of all the other flags.

I think WIT should be changed to allow expressing named flag values that are just combinations of zero or more existing flags by allowing assigning flag values to either 0 or the bitwise or of one or more other named flags:

flags colors {
    black = 0,
    red,
    green,
    yellow = red | green,
    blue,
    magenta = red | blue,
    purple = magenta,
    cyan = green | blue,
    white = red | green | blue,
}