add swig decoder

[AzothZephyr]

this change adds a decoder for the swig program and is generated using the idl from this pr.

[KellianDev]

looks good. please just add the decoder to the workspace cargo file, to the readme and to the publish/yank scripts. then i will merge.

[KellianDev]

also, should get CI to go through ofc @AzothZephyr

[AzothZephyr]

gm,

i have submitted the requested changes. apologies i missed them the first go around.

as far as CI goes, i cleared the simple ones but the swig decoder was generated using the carbon cli. we might want to either review how the cli is generating the decoder to match the linters or adjust the machete and clippy tests if they're failing on output from the tool.

to get this over the line, i can go through and resolve the clippy issues but machete is complaining about packages that are in use. see message below which occurs when i remove base64 as a dependency from the swig decoder.

❯ cargo build
error: failed to load manifest for workspace member `/Users/azothzephyr/unboxed/carbon-az/decoders/swig-decoder`
referenced via `decoders/*` by workspace at `/Users/azothzephyr/unboxed/carbon-az/Cargo.toml`

Caused by:
  failed to parse manifest at `/Users/azothzephyr/unboxed/carbon-az/decoders/swig-decoder/Cargo.toml`

Caused by:
  feature `graphql` includes `dep:base64`, but `base64` is not listed as a dependency