Wren 0.6.18

Wren 0.6.18 fixes a use of freed memory that Linux revealed, and brings the Linux build to the point where it runs.

This is still a beta, under development, and Windows-only: the Linux build is not released yet.

performance.getEntriesByType("navigation") read freed memory. The strings of the navigation entry were freed before they were turned into JSON. The Windows heap left them in place, so nothing showed; on Linux they were overwritten, and GitHub's web-vitals code failed with JSON.parse: bad escape, which left its repository page on "Unable to load page". Fixed.
JSON.parse errors say where they stopped, with the text around the position.
Wren links and runs on Linux (checked under WSLg on NixOS): the window, the start page, wren ctl, and GitHub's repository page. Nothing to download yet: ./build.sh deps and ./build.sh wren build it, and docs/portability.md lists what was checked and what was not.
A new tab no longer puts the keyboard in the address bar (browser.newtab.focus_address brings it back): Ctrl+T opens the new tab page, and the list of frequent pages does not drop over it.
The latest gui_toolkit.

#Wren 0.6.18