feat: Finset builder notation

[YaelDillies]

Define elaborators (but no delaborators) to elaborate the following notations to a Finset:

In Data.Finset.Basic,

• {x ∈ s | p x}

In Data.Fintype.Basic,

• {x | p x}
• {x : α | p x}
• {x ∉ s | p x}
• {x ≠ a | p x}

In Order.LocallyFinite.Basic,

• {x ≤ a | p x}
• {x ≥ a | p x}
• {x < a | p x}
• {x > a | p x}

The general heuristic for deciding whether to elaborate a given notation as a Set or Finset is:

• Check whether the expected type is Finset ?α.
  • If it is, elaborate as a Finset.
  • If it isn't, check whether the expected type of s in the notation is Finset ?α.
    • If it is, elaborate as a Finset.
    • If it isn't or there is no s in the notation, elaborate as a Set.

There is currently a gotcha that elaboration to Set is highly preferred because we can't postpone metavariable and override the set elaborator without breaking many existing uses of set builder notation. See Zulip.

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[YaelDillies]

The new notations here intersect a lot with the notations introduced by #6795, so I'm thinking I should be creating a new syntax category setBinder instead of reusing the existing binderPred.

[github-actions]

PR summary a0b1d241ee

Import changes

No significant changes to the import graph

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Declarations diff

+ elabFinsetBuilderIxx
+ elabFinsetBuilderSep
+ elabFinsetBuilderSetOf
+ elabSetBuilder
+ knownToBeFinsetNotSet

You can run this locally as follows

## summary with just the declaration names:
./scripts/no_lost_declarations.sh short <optional_commit>

## more verbose report:
./scripts/no_lost_declarations.sh <optional_commit>

[YaelDillies]

The new notations here intersect a lot with the notations introduced by #6795, so I'm thinking I should be creating a new syntax category setBinder instead of reusing the existing binderPred.

Given that this is completely orthogonal to the current PR, I will do that in a later PR.