types: add enablePreview to context

[TheAlexLichter]

Somehow the enablePreview function was missing in the Context type.
As it is only available in plugins, I'm not sure if there is a better way to add it. Open for feedback here!

[atinux]

Thanks @manniL

It is a bit tricky indeed since it lives only for static target also.
We also inject $preview, see https://github.com/nuxt/nuxt.js/blob/9681a8937d0c973bc685b757faa686023a4536fc/packages/vue-app/template/index.js#L235-L240

[TheAlexLichter]

True! Should add that in the same PR as well.
Unfortunately, you can't mark functions as optional while objects can be.

[codecov-io]

Codecov Report

Merging #8638 (17365dd) into dev (9ec5d5b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #8638   +/-   ##
=======================================
  Coverage   66.05%   66.05%           
=======================================
  Files          92       92           
  Lines        4010     4010           
  Branches     1104     1104           
=======================================
  Hits         2649     2649           
  Misses       1095     1095           
  Partials      266      266           

Flag	Coverage Δ
unittests	66.05% <ø> (ø)

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Impacted Files	Coverage Δ
packages/webpack/src/config/base.js	65.29% <0.00%> (ø)

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[pi0]

Thanks @manniL @danielroe!