Allow expressions as section headers

[Moderocky]

This is a little proposal for a quality-of-life change, mainly for addons that provide section syntax.

---

Current behaviour

Currently, section headers need to be an effect.

This means addons have to create syntaxes like:

make a new embed and store it in {embed}:
    set the colour of embed to RED
    set the title of embed to "HELLO"
send {embed} to {discordUser}

and

create section with {_proxy}, {_thing} stored in {_var}:
    broadcast "%{_thing}%"

The "store it in" part is necessary since the section header cannot be an expression, so set {x} to a new embed: is currently not possible.

---

Proposed behaviour

I think an expression should be allowed to be a section header as well.
This would allow addons (and Skript itself) to use sensible, regular set... effect rather than needing to create a new effect to store something in a variable.

This would mean that syntax like:

create section with {_proxy}, {_thing} stored in {_var}:
    broadcast "%{_thing}%"

create a gui named "Hello":
            make gui slot 10 with diamond sword
set {gui} to last created gui

could be converted to

set {_var} to section with {_proxy}, {_thing}:
    broadcast "%{_thing}%"

set {_gui} to a gui named "Hello":
    make gui slot 10 with diamond sword

Naturally, there would be occasions where somebody tried to put two section header expressions on the same line, which wouldn't be manageable. This should throw a parse error.