[Feature] Make declarations of function to be POSIX compliant

[Ph0enixKM]

Is your feature request related to a problem? Please describe.
Currently functions generated by Amber are only bash compliant. Let's use the POSIX syntax for declaring functions.

Describe the solution you'd like
Change from:

function foo() {
	# ...
}

to:

foo() {
	# ...
}

Describe alternatives you've considered
N/A

Additional context
This problem is related to the milestone to support sh

[Mte90]

If we implement this we need to add also a flag to Amber to define the bash version to support

[Ph0enixKM]

@Mte90 the POSIX version is supported by Bash and SH. Migrating to the POSIX version will not affect the existing bash compatibility

[Mte90]

I was trying to understand where in the code write function to remove it but I don't find it

amber/src/modules/function/invocation.rs

Line 37 in 042ca2a

syntax_name!("Function Invocation");

[Ph0enixKM]

@Mte90 it's right here:

amber/src/modules/function/declaration.rs

Line 257 in 2e34e15

result.push(format!("function {name} {{"));