Introduce Reduce keyword for stream reduction operations

[flyingsilverfin]

Usage and product changes

We introduce the missing reduce keyword for operations like min/max/count aggregations, as well as first() and check. However, we do not require the reduce keyword for function return statements:

match
  ...
reduce count($x);

in a function would be;

define 
fun test(...) -> long:
  match ...
  return count($x);

We also allow trailing commas throughout the grammar, though they are ignored, to allow the user to generate queries more simply:

match
  $x, isa person, has name $n, ... ; #equivalent to the user-friendly syntax: $x isa person, has name, ...;

Implementation

[typedb-bot]

PR Review Checklist

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Trivial Change

• [ ] This change is trivial and does not require a code or architecture review.

Code

• [ ] Packages, classes, and methods have a single domain of responsibility.
• [ ] Packages, classes, and methods are grouped into cohesive and consistent domain model.
• [ ] The code is canonical and the minimum required to achieve the goal.
• [ ] Modules, libraries, and APIs are easy to use, robust (foolproof and not errorprone), and tested.
• [ ] Logic and naming has clear narrative that communicates the accurate intent and responsibility of each module (e.g. method, class, etc.).
• [ ] The code is algorithmically efficient and scalable for the whole application.

Architecture

• [ ] Any required refactoring is completed, and the architecture does not introduce technical debt incidentally.
• [ ] Any required build and release automations are updated and/or implemented.
• [ ] Any new components follows a consistent style with respect to the pre-existing codebase.
• [ ] The architecture intuitively reflects the application domain, and is easy to understand.
• [ ] The architecture has a well-defined hierarchy of encapsulated components.
• [ ] The architecture is extensible and scalable.