[webview_flutter] Update webview platform interface with new methods for running JavaScript.

[BeMacized]

This PR adds two new platform interface methods, runJavaScript and runJavaScriptForResult, to supersede evaluateJavascript.

This is meant as a solution to the issue mentioned in flutter/flutter#66318, and was discussed offline with @mvanbeusekom and @stuartmorgan.

Relevant issue:

• [webview_flutter] Exception- Error Domain=WKErrorDomain Code=5 "JavaScript execution returned a result of an unsupported type" on iOS 14.  flutter#66318

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran [the auto-formatter]. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the [CLA].
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[BeMacized]

@mvanbeusekom @stuartmorgan
We discussed replacing evaluateJavascript with two new methods, one when no return value is expected and one for where there is. However, while building this I get the feeling we can just replace it with one, as both new methods ignore WKError 5 anyways.

Let me know what you think about that and I can make a quick refactor to merge them if needed.

[mvanbeusekom]

In case of the runJavascriptWithForResult we are clearly expecting a result, so here we should not ignore the WKError 5 in my opinion as this means the developer supplied Javascript which doesn't return a result and we should let them know.

[BeMacized]

@mvanbeusekom Gotcha, I'll quickly fix that

[BeMacized]

PR feedback has been implemented.

All cases of "JavaScript" have been refactored to "Javascript" in this and the other PRs to maintain consistency with existing names for other methods & classes.

[stuartmorgan-g]

LGTM with nits.

[mvanbeusekom]

LGTM

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

• The status or check suite ios-platform_tests has failed. Please fix the issues identified (or deflake) before re-applying this label.