[REQ] Rework logic syntax

[mhatzl]

The current logic syntax has this strange interplay between parameters and body content.
Instead, it would be better to encapsulate Unimarkup content if used as parameter.
e.g.

{$macro my_macro(param1: heading := [[# Default heading]]): heading := [[
	{$param1} in some section
]]}

{$macro add_one(param1: int): int := param1 + 1}

{$if my_value = 1 [[
	Content One
]] else if my_value = 2 [[
	Content Two
]] else [[
	Content {$my_value}
]]}

[mhatzl]

There should also be no differentiation between memorables and macros.
Meaning both should start with {$.

[mhatzl]

If a macro is called, it should also be either using named parameters, or positional parameters.
Mixing them just creates very complex rules and hard to read content.

[mhatzl]

The return type of a macro should be set after a -> arrow, because it resembles similarity to functions.
Functions in scientific fields are often described using a x b -> c, with c being the result.

It also helps to better separate the return type from parameters, and Rust has a similar syntax.

Example:

{$macro my_macro(param1: int) -> int :=
  param1 + 2
}

[mhatzl]

Because Unimarkup syntax is now only considered inside [[ ]], it is possible to use { } for scoping and not be taken as attribute blocks. This makes it easier to create if/else-statements that have no Unimarkup content.

Example:

{$if my_value = 1 {
  my_value += 1
} else if my_value = 2 {
  my_value += 2
} else {	
  my_value := 1
}}

Note: Commets should still work inside non-Unimarkup logic context using ;;

It should be possible to set [[ ]] directly instead of { } to prevent unneeded scoping.

If possible, it should not be required to use ; to separate statements.
In this case it should also not be allowed at all to prevent style conflicts.

[mhatzl]

Maybe instead of [[ ]], using {[ ]} would make it possible to use { } if no markup is set, and otherwise use {[ ]}.
{[ ]} is also better to differentiate from text blocks/boxes. because brackets are surrounded by curly braces.

Maybe even enforce that no space is between bracket and curly braces?
Feedback about readability is needed here.

Example:

{$macro my_macro(param1: heading := [[# Default heading]]) -> heading {[
  {$param1} in some section
]}}

{$macro add_one(param1: int) -> int {
  param1 + 1
}}

{$if my_value = 1 {[
  Content One
]} else if my_value = 2 {[
  Content Two
]} else {[
  Content {$my_value}
]}}