[Developer] Improving UX for TSV editor

Using Keyman Developer 12.0.35.0 beta, I was looking to open a `.tsv` file to use the TSV editor.

From the menu: File --> Open, I had to change filetype to "All files".

1. Should the default filetype "Keyman source files" also add `.model.ts` and `.tsv`?

2. Once I open the .tsv file, the comment
```
# WORD FREQUENCY (Optional)NOTES
```
doesn't quite render the tabs