Fix default Unit property on Component

[mc1098]

Removes the default () unit property when no properties are
specified in the Component definition.

Removes the above assumption in the view macro codegen.
Removes the UnitBuilder struct and Unit prop impl from sycamore-core.

Fixes #415

[codecov]

Codecov Report

Merging #431 (fbf35f4) into master (5f58fe3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #431   +/-   ##
=======================================
  Coverage   64.96%   64.97%           
=======================================
  Files          52       52           
  Lines        8278     8268   -10     
=======================================
- Hits         5378     5372    -6     
+ Misses       2900     2896    -4     

Impacted Files	Coverage Δ
packages/sycamore-core/src/component.rs	68.18% <ø> (+10.48%)	⬆️
packages/sycamore/src/builder.rs	23.11% <ø> (ø)
packages/sycamore-macro/src/component/mod.rs	95.95% <100.00%> (-0.07%)	⬇️
packages/sycamore-macro/src/view/codegen.rs	96.22% <100.00%> (-0.03%)	⬇️

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[lukechu10]

Looks great thanks! Not sure why I did that in the first place but it's definitely cleaner now.