Fix boolean attributes in presence of spread attributes

[Conduitry]

Fixes #3764, with the assumption that #3750 may mean that it would become even more impractical to ship attribute-to-property tables along with the runtime.

In short:

• For each boolean attribute (more precisely, for each attribute with attribute_lookup metadata) given for a node that has at least one spread, the compiler will emit the appropriate property key in the object in the spread arrays. For example, { readOnly: whatever } instead of { readonly: whatever }.
• In SSR mode, the corresponding object in the spread array now contains { readonly: whatever || null } instead of {readonly: whatever } for boolean attributes, to ensure that the spread() function does not serialize falsy attributes.

Bonus SSR bugs I bumped into and fixed while addressing this:

• {...{ foo: null }} was getting serialized as foo="null" because spread() was checking for strict equality with undefined. It's now checking for loose equality with null.
• Attributes had to be lowercase for the special logic for value, class, or any boolean attribute to take effect. These checks are now done case-insensitively.

[Conduitry]

Bonus bonus fix (thanks @mrkishi):

<input placeholder='foo' {...{ placeholder: null }}>

now produces <input> rather than <input placeholder='null'>.

[Conduitry]

gh actions seem to be having a bad time. I'll re-run later today and see if I can get that ol green checkmark before merging this.