[FEATURE] Adds inline if and unless keywords

[pzuraq]

Directly adds the inline versions of the if and unless keywords to
the VM. It does this by adding a new IfInline opcode, which is
combined with a new Not opcode that inverst the condition in the case
of unless. This also allows us to remove the Unless opcode and
rewrite it in terms of the If opcode in general. Since unless is
generally much less common of an opcode, this shouldn't be an issue
performance-wise, and we benefit from having a simpler set of
wireformat/opcodes.

[lifeart]

wondering, could we have not instead of unless ?

• it's widely used outside ember (comparing to unless)
• easier to understand (for non-english speakers)
• less symbols to write

downsides:

{{#unless this.foo}} {{/unless}} has to be replaced with {{#if (not this.foo) }} {{/if}},
but, it's bring mutch clarity into logic (IMHO)

one more case - have if-not instead of unless (more clarity)

point is - unless wery "rare" word and adding more mental overhead for non-native speakers, also, used 20 times less in the world, comparing to if not

[pzuraq]

@lifeart yeah, that's actually my preferred usage. (not) is currently in RFC, and the thinking is that this could be how it works under the hood. We need to decide if not is a keyword or a helper first though, so we'll see.

As for removing unless, it already exists in the language so it would need an RFC to remove it. This change means we don't have to pay for it conceptually at least, in the future we could discuss removing.