Use the same profile name for both Cargo and wasm-pack

[srussvoll]

When running wasm-pack with a custom profile (wasm-pack build . --profile custom_profile_name), the Cargo profile with name custom_profile_name will be used, but wasm-pack will use a profile called custom.

More concisely,
Cargo will use [profile.custom_profile_name], whereas
wasm-pack will use [package.metadata.wasm-pack.profile.custom]

This PR makes wasm-pack use the custom profile name provided with the --profile option so that
wasm-pack will now use [package.metadata.wasm-pack.profile.custom_profile_name].

Closes #1488.

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[srussvoll]

To my knowledge the custom profile is currently undocumented, and the feature has not yet been released. But since some users is likely using the git repository to install wasm-pack, I added a deprecation warning for the custom profile. Is this desirable?