Add "%%" to noBreakInfixOps to fix soundness bug with custom %% operator

The custom %% operator (and the built-in F# quotation splice operators %%
and %%) can be interpreted by the F# parser as unary prefix operators when
they appear at the start of a new line. This caused Fantomas to produce
invalid F# code by placing the operator on a new line.

The fix adds "%%" to the noBreakInfixOps set, which keeps the operator on
the same line as its left-hand-side expression (with the right-hand-side
indented below if needed), matching the existing behaviour for %, =, <, >.

Fixes #2107

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

src/Fantomas.Core.Tests/OperatorTests.fs

@@ -493,6 +493,29 @@ let hasUnEvenAmount regex line =
      - Regex.Matches(line, "\\\\" + regex).Count) % 2 = 1
 """
 
+[<Test>]
+let ``custom double-percent operator stays on same line as lhs when expression is too long, 2107`` () =
+    formatSourceString
+        """
+let inline (%%) x y = ((x % y) + y) % y
+let aVeryLongVariableNameThatForceLineBreaking = 0
+let a = (if aVeryLongVariableNameThatForceLineBreaking = 0 then 1 else -1) %% 4
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+let inline (%%) x y = ((x % y) + y) % y
+let aVeryLongVariableNameThatForceLineBreaking = 0
+
+let a =
+    (if aVeryLongVariableNameThatForceLineBreaking = 0 then
+         1
+     else
+         -1) %% 4
+"""
+
 [<Test>]
 let ``parameter after multiline string, 783`` () =
     formatSourceString

src/Fantomas.Core/CodePrinter.fs

@@ -5,7 +5,7 @@ open Fantomas.Core.Context
 open Fantomas.Core.SyntaxOak
 open Microsoft.FSharp.Core.CompilerServices
 
-let noBreakInfixOps = set [| "="; ">"; "<"; "%" |]
+let noBreakInfixOps = set [| "="; ">"; "<"; "%"; "%%" |]
 let newLineInfixOps = set [ "|>"; "||>"; "|||>"; ">>"; ">>=" ]
 
 let rec (|UppercaseType|LowercaseType|) (t: Type) : Choice<unit, unit> =