Adds strict text updates

[jacwright]

{@strict text} will only update the text node if the text node's value is not equal. E.g. instead of

text.data = data;

strict will use

if (text.data !== data) text.data = data;

This is only needed (from what I can tell) in contenteditable situations where the user can update the text in the Node outside of Svelte's data source. In this situation, Svelte needs to check if the value is equal so it doesn't overwrite a text composition in action. This applies to writing letters such as é and in languages such as Thai.

See https://v3.svelte.technology/repl?version=3.0.0-beta.22&gist=93c7a0c8399348d8e9a86964aef9f8fb for a bit more about this.

[codecov-io]

Codecov Report

Merging #2341 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2341   +/-   ##
=======================================
  Coverage   91.83%   91.83%           
=======================================
  Files           1        1           
  Lines          49       49           
=======================================
  Hits           45       45           
  Misses          4        4

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[Rich-Harris]

I wonder whether the cost of checking the current text.value is low enough that this should just be the default?

[jacwright]

I suppose since it isn't a unique part of a component that it won't increase the file size except for the few bytes. I would think it is better than increasing the API footprint.

[jacwright]

Closed in favor of #2357.