Introduce MSRV policy

[daxpedda]

This PR introduces a MSRV policy.

Library crates have an MSRV of v1.57 and changes to the MSRV are accompanied by a minor version bump.
The following crates are included in this policy:

• wasm-bindgen
• js-sys
• web-sys
• wasm-bindgen-futures
• wasm-bindgen-test
• wasm-bindgen-macro
• wasm-bindgen-test-macro
• wasm-bindgen-macro-support
• wasm-bindgen-backend
• wasm-bindgen-shared

CLI crates and their support libraries have an MSRV of v1.76, due to being dependent on the wasm-tools ecosystem. The MSRV is only guaranteed for a specific version, so it is only accompanied by a patch version bump.
The following crates are included in this policy:

• wasm-bindgen-cli
• wasm-bindgen-cli-support
• wasm-bindgen-wasm-interpreter
• wasm-bindgen-threads-xform
• wasm-bindgen-multi-value-xform
• wasm-bindgen-externref-xform
• wasm-bindgen-wasm-conventions

Fixes #3114.
Fixes #3918.

[Liamolucko]

Library crates have an MSRV of v1.57 and changes to the MSRV are accompanied by a minor version bump.

That's a lot stronger of a policy than I would have expected: since all of those crates are at 0.x.y, a minor version bump is effectively a major version bump, and so we'll be stuck at an MSRV of 1.57 until we make a breaking release.

[daxpedda]

Library crates have an MSRV of v1.57 and changes to the MSRV are accompanied by a minor version bump.

That's a lot stronger of a policy than I would have expected: since all of those crates are at 0.x.y, a minor version bump is effectively a major version bump, and so we'll be stuck at an MSRV of 1.57 until we make a breaking release.

Yep.
I don't see us really raising the MSRV considering we don't really have any dependencies.

Do you disagree?