Properly set boolean attributes on custom elements

[geoffrich]

Fixes #5951

When given a standalone boolean attribute on a custom element, the compiler was setting the corresponding property to empty string, which is falsy. This made it seem as the property was not being applied.

For example, using the following in a Svelte template sets the raised property of the mwc-button custom element to ''. Instead, it should be set to true.

<mwc-button raised />

With this change, the compiler will set boolean properties on custom elements to true when no value is provided.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[vospascal]

@geoffrich nice work, is this already released? (ah i see in 3.38)
also one more question if you look at https://github.com/webcomponents/custom-elements-everywhere/blob/master/libraries/svelte/src/components/ComponentWithProperties.svelte
this is from svelte to webcomponents, wouldnt it be nice to also support somehow the pasing of array and object properties?
This would make the svelte custom-elements work with other things nicely

[vospascal]

@tanhauhau could svelte support something like this ?
https://lit-element.polymer-project.org/guide/properties

LitElement has a default converter which handles String, Number, Boolean, Array, and Object property types.

prop1: { type: String },
prop2: { type: Number },
prop3: { type: Boolean },
prop4: { type: Array },
prop5: { type: Object }