Simpler codegen

[Rich-Harris]

This stops well short of the idea described in #652 (which I started investigating, but concluded it would probably add more complexity and indirection than it removes), but I think it goes a long way towards making the code-that-generates-the-code more readable.

// before
this.builders.unmount.addLine(
  `${this.generator.helper('detachNode')}( ${name} );`
);

// after
this.builders.unmount.addLine(
  `@detachNode( ${name} );`
);

Essentially @foo means 'alias foo, and register it as a helper if there's a helper called foo'. Similarly, for block-level aliases, as opposed to component-level, we can use #foo.

[Conduitry]

It looks like this has problems if the original component has things in it that look like these special identifiers. Compiling just the string @foo results in text = createText( "\\foo" );

Compiling {{'@foo'}} results in text = createText( text_value = '@foo' ); which seems fine.

Compiling #foo results in text = createText( "\#foo" ); which isn't ideal but technically works.

[Rich-Harris]

Good catch! Should be fixed now