Implemented function expressions

[Razican]

This also adds the name property to functions. It fixes #266 and makes progress towards #12.

There is one extra thing that is not solved though:

let a = function() {};
a.name;

This should show a, but this is currently not happening. I guess that the solution is to change the assignment expression to change the name of the function.

I duplicated most of the execution code for declarations/expressions, though, this can probably be improved in the future. I used this to set the name of the function:

val.set_field_slice("name", to_value(name.clone()));

This is not super spec compliant, but do you think it's good for now?

[github-actions]

Benchmark for de95f3e

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Test	PR Benchmark	Master Benchmark	%
Create Realm	485.0±22.54µs	486.3±13.93µs	100%
Expression (Lexer)	1990.8±75.35ns	1981.3±19.24ns	100%
Expression (Parser)	5.3±0.13µs	5.1±0.11µs	104%
Fibonacci (Execution)	3.3±0.15ms	3.3±0.03ms	101%
For loop (Execution)	503.6±19.15µs	505.9±8.71µs	100%
For loop (Lexer)	5.4±0.13µs	5.5±0.15µs	99%
For loop (Parser)	14.5±0.17µs	14.3±0.26µs	101%
Hello World (Lexer)	959.5±9.42ns	969.6±12.91ns	99%
Hello World (Parser)	2.3±0.06µs	2.4±0.10µs	97%
Symbols (Execution)	514.4±18.44µs	513.7±13.37µs	100%
	undefined	undefined	100%

[github-actions]

Benchmark for decbee6

Click to view benchmark

Test	PR Benchmark	Master Benchmark	%
Create Realm	473.6±21.62µs	466.7±18.62µs	101%
Expression (Lexer)	1929.9±73.77ns	1912.0±69.85ns	101%
Expression (Parser)	5.2±0.16µs	4.9±0.15µs	104%
Fibonacci (Execution)	3.2±0.07ms	3.2±0.05ms	100%
For loop (Execution)	485.1±9.86µs	489.8±15.09µs	99%
For loop (Lexer)	5.2±0.19µs	5.2±0.09µs	101%
For loop (Parser)	13.9±0.33µs	13.8±0.38µs	101%
Hello World (Lexer)	944.3±47.82ns	935.7±47.56ns	101%
Hello World (Parser)	2.2±0.05µs	2.2±0.03µs	99%
Symbols (Execution)	500.4±11.00µs	494.6±10.50µs	101%
	undefined	undefined	100%