Fix #520 Red underline upon sentence completion

[icburns]

Addresses #520

This PR addresses spellcheck highlighting at the ends of sentences when a valid word is followed by a period, and alters the existing special handling around words that contain periods.

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Issue:

Valid words were being erroneously flagged by the spellchecker when followed by a period.

Reproduction on OpenBoard 1.4.5

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Resolution:

Valid words followed by periods are no longer flagged. Invalid words followed by periods are still flagged, and the intent behind the existing logic to flag and suggest valid words on either side of a period within a token should not be impacted by this change (see below).

Testing on Pixel 5 Android 13

Valid words on either side of an interceding period are suggested with a space as separator

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Additional Considerations for CHECKABILITY_CONTAINS_PERIOD:

• Empty words are considered invalid. In CHECKABILITY_CONTAINS_PERIOD, when a period was the first or last index of text, splitting that text on periods caused suggestions to consider potentially valid words to be invalid. For example, "this.example." should suggest "this example" or "this example.", but no suggestions were being given due to the trailing empty string resulting from the split operation being considered invalid. This PR modifies this behavior such that empty split tokens will not be considered toward word validity when generating suggestions for text containing periods.

• Single word sentences (like "Groovy.") were being flagged as invalid since the word only exists in the main dictionary as lower case, but the validity check within CHECKABILITY_CONTAINS_PERIOD sent text over as is. This PR adds, specific to the period checkability logic, a fallback validity check for words set to lower case.

[mintsoft]

Yes! The old behaviour of this drives me insane

[Switch123456789]

What's up with this PR? Why isn't it merged yet? I am dealing with this issue for almost a year now (it's the stock keyboard of iodeOS) and it is just SOOO unbearable. I have no choice but to abandon OpenBoard now, which is a shame considering all the work you put in and how awesome the keyboard COULD be without this bug.