feat(svelte): warn on unquoted class attributes

[tivac]

Description

Warn users whenever an unquoted class={css.foo} is discovered.

Motivation and Context

The svelte preprocessor can create subtle bugs in cases where the user types something like class={css.foo}, this is fine assuming that css.foo is replaced with a single class but fails hard if there's any composition involved.

Single-class case ✔

-<div class={css.foo}>
+<div class=foo>

which is treated by the browser as <div class="foo">

Multi-class case 💀

-<div class={css.fooandbar}>
+<div class=foo bar>

which is treated by the browser as <div class="foo" bar>, which is definitely a problem.

More background on unquoted attributes is available from Mathias Bynens.

How Has This Been Tested?

Added tests!

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #690 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #690      +/-   ##
==========================================
+ Coverage   98.44%   98.45%   +0.01%     
==========================================
  Files          45       45              
  Lines        1157     1168      +11     
  Branches      179      181       +2     
==========================================
+ Hits         1139     1150      +11     
  Misses         15       15              
  Partials        3        3

Impacted Files	Coverage Δ
packages/svelte/svelte.js	100% <100%> (ø)	⬆️

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