chore!: Require types of globals to be specified

[michaeljklein]

Description

Problem*

Resolves global type inference issues in https://github.com/noir-lang/noir/pull/6469/

Summary*

The approach in https://github.com/noir-lang/noir/pull/6469/ is to evaluate all globals as comptime globals, but that interacts poorly with polymorphic types, e.g. the type of x in let x = 3.

Additional Context

Documentation*

Check one:

• [ ] No documentation needed.
• [x] Documentation included in this PR.
• [ ] [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• [x] I have tested the changes locally.
• [x] I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions[bot]]

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[github-actions[bot]]

FYI @noir-lang/developerrelations on Noir doc changes.