**Bug**
For long/not-terminated frontmatter in a markdown file, we can currently hang the process while trying match it using a regular expression

**Fix**
Use a more efficent regexp to do this.

Fixes #19375
Related #19863

Fixes #19863
Related #19375

The setting will now correctly open a new window when no file is specified, but if
a file is specified it will reuse. This is so opening via the file manager will reuse
the window.

Fixes #19775

- Do not use scheme for files
- Use scheme for non files

Fixes #19740: Find and replace capture group/backreference inserts `undefined` instead of empty string

Implement modal message API

fixes #19362

Fixes #19766
Related #19474