detect unused/misplaced components

[Rich-Harris]

Ref #1039. This issues a) a compile time warning if unused components are detected, and b) a dev mode run time warning if missing data properties are found on components, on the probability that the component author intends for those components to be used with <:Component>

[codecov-io]

Codecov Report

Merging #1050 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1050      +/-   ##
==========================================
+ Coverage   92.02%   92.04%   +0.02%     
==========================================
  Files         121      121              
  Lines        4376     4388      +12     
  Branches     1410     1415       +5     
==========================================
+ Hits         4027     4039      +12     
  Misses        148      148              
  Partials      201      201

Impacted Files	Coverage Δ
src/generators/server-side-rendering/index.ts	95.77% <ø> (ø)	⬆️
src/validate/index.ts	88.67% <100%> (+2.01%)	⬆️
src/generators/dom/index.ts	95.48% <100%> (+0.05%)	⬆️
src/validate/html/validateElement.ts	89.13% <100%> (+0.24%)	⬆️

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[tomcon]

Had a silly error where I'd cut and pasted a ActiveItems component like so:

<ActiveItems><ActiveItems>

(i.e. didn't close off the component tag)

rollup was watching and re-compiled it ok but the runtime error wasn't the best and it took quite some time to track this one down

Uncaught TypeError: Cannot read property 'default' of undefined
    at Object.mount [as m] (App.html:24)
    at new App (App.html:24)
    at main.js:6
    at main.js:22

[Rich-Harris]

@tomcon can you create a repro please (in a separate issue, as it's not strictly related to this PR)? I can't seem to get the same error. Thanks