Add support for Alternate Registries

[BarbossHack]

Add support for alternate registries, as discussed in #217 (comment) :

• Read alternate registries from ~/.cargo/config.toml (or ~/.cargo/config for legacy) if any.
• Read registry tokens from ~/.cargo/credentials.toml if any.
• Support registry authentification with cargo:token credentials.
• Does not support registry authenfication with cargo:libsecret, cargo:macos-keychain, cargo:wincred. Support may be added later.
• Decorator (mouseover popup) fields View Crate, Check review and Doc are hidden for crates that are on alternate registries.
• Default behavior is to ignore crates that have an alternate registry :
  • If a crate have an alternate registry, but it could not be found in config.toml, just ignore it without error, and do NOT use indexServerURL as a fallback registry.
  • If the registry is found, then fetch versions on this registry.
• I've added a script (./sample/alternate-registry.sh) to facilitate testing with alternate registries, for development purposes only.

Other modifications :

• I removed src/api/crates-index-server.ts as it was not used anymore
• I removed isSparseCompatible() as it was not used anymore

Fixes #213
Fixes #182