Potential performance improvements of the parser: delay type widening in `parse_list_expression` and `parse_table_expression`

Basics

• I have done a basic search through the issue tracker to find similar or related issues.
• I have made myself familiar with the available features of Nushell for the particular area this enhancement request touches.

Related problem

let foo: list<any> = [1 2 3 a b c] currently triggers type widening when parsing the list expression, then the widened list<oneof<int, string>> is abandoned because there's an explicit type annotation list<any> in the assignment.

I would say most of the widened types are useless, and should only be computed when being checked against something non-trivial.

Describe the solution you'd like

https://github.com/nushell/nushell/pull/17575#issuecomment-4017986692

That strategy seems better suited for new-nu-parser.
Nu-parser probably needs more tweaks.

Describe alternatives you've considered

No response

Additional context and details

No response

Start with parse_list_expression and parse_table_expression in the Nu-parser and new-nu-parser implementations, then read the strategy discussed in PR #17575. Determine how explicit annotations such as list should avoid unnecessary type widening, and verify that the resulting parser behavior matches the intended deferred-checking strategy.