Refactor to use a dedicated MatchPattern enum instead of reusing Expression

[jfecher]

Problem

Match patterns introduced in #7243 parse into Expressions for ease of implementation but we should probably refactor this into a dedicated Pattern type in the future.

Happy Case

A dedicated Pattern type would:

• Eliminate the need for a _ => error invalid pattern case when matching expression types that are not supposed to be part of a pattern. E.g. a Lambda.
• Allow us to add pattern syntax that is invalid in other expressions like "or" patterns using | or alias patterns using @.
• Likely require significantly more code since we'd have a new recursive type to elaborate, handle in the language server, create a resolved variant of, etc.

Workaround

None

Workaround Description

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Additional Context

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Project Impact

None

Blocker Context

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