feat(sidebar-filter): measure when user types in the filter

[caugner]

Summary

(MP-1007)

Problem

We measure when users focus the sidebar filter, but we don't measure when users type in it.

Solution

Add a measurement on the first type.

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How did you test this change?

Ran yarn && yarn dev with Glean debugging enabled (REACT_APP_GLEAN_CHANNEL, REACT_APP_GLEAN_DEBUG and REACT_APP_GLEAN_ENABLED).

1. Opened http://localhost:3000/en-US/docs/Learn/HTML
2. Focused the sidebar filter (and observed the sidebar_filter_focus measurement).
3. Started typing (and observed a single instance of the new sidebar_filter_typed measurement).
4. Unfocused, focused again and continued typing (without observing any measurement).
5. Cleared the sidebar filter, focused again (and observed another sidebar_filter_focus measurement).
6. Started typing again (and observed another sidebar_filter_typed measurement).