Mixing RTL and LTR text bugs

[GaryQian]

We are seeing strange behavior when mixing RTL and LTR text.

Notably:

• Type arabic, Type chinese, Type arabic, switch to chinese keyboard, delete. The arabic will delete all at once.
• Emojis work strangely when mixing with RTL and LTR.
• Caret positioning is not consistent.

There are a few other minor issues that I am not clear on how to describe.

[GaryQian]

After some investigation, when mixing LTR and RTL text, the ParagraphStyle's directionality is LTR. This is causing the space to be inserted at the wrong end of the arabic span. We should handle arabic spaces in a RTL manner even if the paragraph direction is LTR. The correct solution to this is to change the paragraph to RTL when dealing with RTL text, however, when arabic snippets are inline with primarily LTR text, this may not be possible.

[GaryQian]

Caret positioning has vastly improved with flutter/engine#9489

The primary remaining issue is handling deletion properly with keyboards of opposite direction.