Constrain Structs.Map key types

[bagel786]

Description

Tightens the typing of Phaser.Structs.Map (items 1-2 of #7323):

• Constrains the K generic to string | number, so indexing the Map with an object or other unsafe-coercion keys is now a type error.
• Removes the shadowing K/V generics from setAll (they disabled type safety entirely); elements is now typed as the nested-array pair form (K|V)[][], which is the closest jsdoc can get to the ideal [K, V][] tuple type until declarations support tuples.
• The constructor's elements parameter is now actually optional in the types, matching its documented and runtime behavior.
• get now includes undefined in its return type, matching its JSDoc.

Generated declarations were inspected (the Map class declaration now reads class Map<K extends string|number, V>, constructor(elements?: (K|V)[][], setAll(elements?: (K|V)[][]): this, get(key: K): V|undefined) but are not included in this PR because the current generator rewrites tens of thousands of unrelated lines.

Fixes #7323 (items 1 and 2)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have run npm run ts and no errors are present.
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I solved any possible warnings that npm run ts has generated that are related to my code to the best of my knowledge.
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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