[TextInput] Add TextInputType.webSearch (#15762)

[stonemaster]

This PR adds the engine part to add TextInputType.webSearch that allows to show a keyboard with ready access to a "." key on iOS. On Android this is re-mapped to url which shows the same behaviour as webSearch on iOS. This fixes issue flutter/flutter#157562.

The flutter PR: flutter/flutter#158323

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 [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is [test-exempt]. See [testing the engine] for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[Renzo-Olivares]

LGTM w/ small nit.

[stonemaster]

I added a new testcase 🎉