Fix ternaries where "true" is a parenthesized variable and "false" is a function expression

[andrewbranch]

Fixes #30635

Decided to try parsing JSDoc function types (e.g. /** @type {function(): void} */) only inside JSDoc per @weswigham’s suggestion, see what breaks, and gather feedback.

Looks like what breaks is the error message on code like

var g: function(number, number): number = (n,m) => n + m;

Previously, the error was JSDoc types can only be used inside documentation comments. With this change, it becomes a series of nasty parse errors and Cannot find name 'function'. 😕

Possible simple options off the top of my head:

• Go with this because people probably don't often try to write code like the snippet above
• Try to disable JSDoc function keyword parsing only when parsing the whenTrue of a conditional expression: super magically edge-casey but keeps all existing nice error messages.

Thoughts?

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EDIT

Updated with a different approach. Typically, arrow functions parse successfully even if they're missing the => token, since in most cases it's still unambiguously an arrow function that's just missing a =>. When parsing the true side of a conditional expression, combined with the fact that function() parses as a type for JSDoc compatibility, it's not unambiguously an arrow function.

So, when the arrow function in-progress has a JSDocFunctionType type, I don't allow a missing => token.