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

This came up in https://github.com/WebAssembly/wasi-gfx/pull/71#discussion_r3358530577

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 https://github.com/WebAssembly/wasi-gfx/pull/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,
}

note: I'm not planning on implementing this myself, so whoever's interested feel free to implement it whenever ready.

Review the linked wasi-gfx discussion and PR 71, then trace how WIT flag declarations are specified and generated. The work is done when named flag values can be zero or combinations of existing flags, including aliases such as all, without allocating new bits.