feat: add structInstFieldDecl syntax category (#6265)

This PR is preparation for changes to structure instance notation in
#6165. It adds a syntax category that will be used for field syntax.

src/Init/Notation.lean

@@ -48,6 +48,11 @@ def tactic : Category := {}
 For example, `let x ← e` is a `doElem`, and a `do` block consists of a list of `doElem`s. -/
 def doElem : Category := {}
 
+/-- `structInstFieldDecl` is the syntax category for value declarations for fields in structure instance notation.
+For example, the `:= 1` and `where a := 3` in `{ x := 1, y where a := 3 }` are in the `structInstFieldDecl` class.
+This category is necessary because structure instance notation is recursive due to the `x where ...` field notation. -/
+def structInstFieldDecl : Category := {}
+
 /-- `level` is a builtin syntax category for universe levels.
 This is the `u` in `Sort u`: it can contain `max` and `imax`, addition with
 constants, and variables. -/

src/Lean/Parser/Term.lean

@@ -387,6 +387,13 @@ use `(_)` to inhibit this and have it be solved for by unification instead, like
     explicitBinder requireType <|> strictImplicitBinder requireType <|>
     implicitBinder requireType <|> instBinder
 
+/-
+Syntax category for structure instance notation fields.
+Does not initialize `registerBuiltinDynamicParserAttribute` since this category is not meant to be user-extensible.
+-/
+builtin_initialize
+  registerBuiltinParserAttribute `builtin_structInstFieldDecl_parser ``Category.structInstFieldDecl
+
 /-
 It is feasible to support dependent arrows such as `{α} → α → α` without sacrificing the quality of the error messages for the longer case.
 `{α} → α → α` would be short for `{α : Type} → α → α`