fix(Theme): Deep merge variables

[miroslavstastny]

Do deep merge of site variables and component variables.

Motivation

Let's have two themes (parent, child) merged.
To resolve styles, follow steps occur

1. child.siteVariables are merged on top of parent.siteVariables to compute siteVariables
2. computed siteVariables are used to compute parent.componentVariables and child.componentVariables, child.componentVariables are merged on top of parent.componentVariables
3. computed componentVariables are used to compute parent.styles and child.styles

That means that if child variables remove a variable from parent and parent styles use that variable, an exception will be thrown.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@7ca52d9). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master   #1907   +/-   ##
========================================
  Coverage          ?   70.5%           
========================================
  Files             ?     884           
  Lines             ?    7794           
  Branches          ?    2254           
========================================
  Hits              ?    5495           
  Misses            ?    2289           
  Partials          ?      10

Impacted Files	Coverage Δ
packages/react/src/lib/mergeThemes.ts	100% <100%> (ø)
packages/react/src/lib/deepmerge.ts	100% <100%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7ca52d9...4468e48. Read the comment docs.

[layershifter]

Please measure it before merging 🤔

[miroslavstastny]

Please measure it before merging 🤔

master:

Example	Avg	Avg diff	Avg Normalized	Avg Normalized diff	Median	Median diff	Median Normalized	Median Normalized diff
ChatWithPopover.perf.tsx	561.7	+3.84%	955.3	955.3	614.07	+27.37%	571.12	571.12
Chat.perf.tsx	540.91	540.91	956.14	+0.09%	482.11	482.11	1164	+103.81%

deep merge branch:

Example	Avg	Avg diff	Avg Normalized	Avg Normalized diff	Median	Median diff	Median Normalized	Median Normalized diff
Chat.perf.tsx	511.91	511.91	969.01	969.01	482.27	482.27	1122.21	+74.7%
ChatWithPopover.perf.tsx	558.12	+9.03%	996.02	+2.79%	609.04	+26.29%	642.37	642.37

My interpretation of the results: there is no measurable perf impact.

[levithomason]

👍