refactor(parser): Remove lexer lookahead in JS let declaration parsing

[therewillbecode]

Part of #11194.

Removes usage of lexer lookahead methods in favour of using the parser lookaheads for parsing let declarations.

[graphite-app]

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #11308 will not alter performance

_{Comparing therewillbecode:refactor/remove-lexer-lookahead-in-decl (05b3e4c) with main (590c27b)}

Summary

✅ 38 untouched benchmarks

[Boshen]

This is on a hot path, but none of our benchmarks exercise this hot path fully.

Tracking SyntaxKind.LetKeyword I found

            case SyntaxKind.LetKeyword:
                if (isLetDeclaration()) {
                    return parseVariableStatement(getNodePos(), hasPrecedingJSDocComment(), /*modifiers*/ undefined);
                }
                break;

So maybe a little refactoring to make the code read better in a follow up PR.