prefer-ternary: convert options to object, add onlyAssignments option

[ipanasenko]

Fixes #1633

Was created with the help from Claude 4.5 Opus

[sindresorhus]

1. [P1] Autofix can introduce let TDZ runtime errors in rules/prefer-ternary.js:329.
   The new declaration-merging fixer rewrites:

let foo;
if (foo) {
	foo = 1;
} else {
	foo = 2;
}

to:

let foo = foo ? 1 : 2;

which throws (Cannot access 'foo' before initialization). This is a behavior regression.

2. [P1] Autofix drops comments between declaration and if in rules/prefer-ternary.js:337.
   fixer.removeRange([declarationRange[0], ifRange[0]]) removes everything between nodes, including standalone comments, for example:

let foo;
// keep me
if (test) { ... }

becomes:

let foo = test ? ... : ...;

with the comment deleted.

3. [P1] Autofix drops TypeScript annotations on merged declarations in rules/prefer-ternary.js:331.
   Using declarations[0].id.name reconstructs only the bare identifier.
   let foo: number; if (...) { foo = 1; } else { foo = 2; } becomes let foo = ...;, losing : number.

[cobaltt7]

My $0.02: I feel like it makes less sense to limit it to only assignments, and I'd prefer that the rule not warn on yield or await. Most of the annoying examples I've seen that I don't want fixed involve await. Typically that means it's also not an assignment, but something like this I still wouldn't like

let output = await (test ? a() : b());

[ipanasenko]

@cobaltt7 in this PR it will be fixed like this:

let output = test ? await a() : await b();

[sindresorhus]

Merge conflict.

__

Incomplete TDZ check — only inspects node.test, not branches

The TDZ guard only checks if the variable appears in the if-condition:

const tokens = sourceCode.getTokens(node.test);
if (tokens.some(token => token.type === 'Identifier' && token.value === variableName)) {
    precedingVariableDeclaration = undefined;
}

This misses cases where the variable is referenced in the assignment branches:

let foo;
if (test) {
    foo = foo + 1;  // reads `foo` (which is `undefined`)
} else {
    foo = 2;
}
// Merged to: let foo = test ? foo + 1 : 2;  ← TDZ error!

The original code is valid (foo is undefined), but the merged version throws because foo is in TDZ during initializer evaluation. The check should also scan consequent.right and alternate.right for references to the variable.

---

TDZ check uses token matching instead of scope analysis

Using sourceCode.getTokens() and matching on token.type === 'Identifier' is imprecise. It would false-positive on property accesses like obj.foo where foo matches the variable name but isn't actually a reference to it. Scope analysis (sourceCode.getScope() + walking references) would be more correct. The current approach is conservative (skips valid merges rather than creating bugs), so it's not critical, but worth noting.

---

onlyAssignments name is misleading

The option also allows return statements, not just assignments. The filter at line 256-261:

if (
    onlyAssignments
    && consequent?.type !== 'AssignmentExpression'
    && consequent?.type !== 'ReturnStatement'
) {
    return;
}

Consider a more descriptive name like onlyValueCapture or at minimum document clearly in the docs that returns are also included (the docs do mention this, but the option name is still confusing).