feat: Flag this/these not agreeing in number with the noun and/or verb

## Description

This is a very common mistake for non-native English speakers whose native language does not have both a "short i" sound (like in "this") _and_ a "long i" sound (like in "these")

## Resources

- https://www.youtube.com/shorts/IEeMBG_aidI
- https://www.youtube.com/shorts/7F-PZUHpwyg
- https://youtu.be/1-wVmfAhq-o
- https://youtu.be/5jQdTpcBqXo
- (text descriptions of the mistake are surprisingly hard to find)

## Examples

Found in the wild:
- **This files is** highly inspired from ...
→ This **file** is highly inspired from ...
→ **These** files **are** highly inspired from ...
- **this is** actually very serious **issues**
→ this is actually **a** very serious **issue**
→ **these are** actually very serious issues
- **This** companies have stocked goods in advance → **These** companies have ...

Start by reviewing the existing grammar-checking rules and tests that handle demonstratives, noun number, and verb agreement. Use the issue's three examples as cases to reproduce, and consider the work complete when the checker flags the incorrect forms without introducing false positives for the valid singular and plural alternatives.

Written by the indexing model from the issue text.