Editor: fold (...) blocks and whole function definitions

Is your feature request related to a problem? Code folding in the built-in editor currently only understands {} and []. Two common constructs have no way to collapse:

• Any multi-line call or argument list that only uses plain parentheses — e.g. a multi-line let(...) definition, object(...) call (see OEP8a, the experimental object() builtin), or any other function/module call whose arguments span several lines.
• A function name(...) = expr; definition. Since function bodies are pure expressions with no braces, even a long, deeply-nested definition (e.g. one built around let(...)) has no single fold point that collapses the whole thing — only its own internal ([ sub-expressions fold individually.

Describe the solution you'd like

• Treat ( and ) the same as the existing {} and [] bracket-depth counting in ScadLexer2::fold().
• Additionally fold the entire span of a function ... = ... ; definition as one region, from the function keyword to its terminating semicolon. Since a function body cannot itself contain a semicolon the first semicolon following the keyword unambiguously terminates it.

Describe alternatives you've considered

I've also considered a more general #region/#endregion-style comment marker (as in VS Code), but that's out of scope here. This issue is only about the two gaps above, which are purely syntax-driven.

Additional context No test infrastructure currently exercises ScadLexer2; a data-driven QtTest suite (src/guitests/TestScadLexerFolding) is included in the linked PR.