devel/electron29: add experimental port of electron 29

Chromium patches obtained from:
FreeBSD ports tree at 6166c62620dcd2a760e4306bd8a33716cdfb6bb0

Changelog: https://www.electronjs.org/blog/electron-29-0

Author: tagattie

Mk/Uses/electron.mk

@@ -108,7 +108,7 @@ _INCLUDE_USES_ELECTRON_MK=	yes
 # Electron uses Node (actually a package manager) for build
 .include "${USESDIR}/nodejs.mk"
 
-_VALID_ELECTRON_VERSIONS=	6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
+_VALID_ELECTRON_VERSIONS=	6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
 _VALID_ELECTRON_FEATURES=	npm prefetch extract rebuild build
 _VALID_ELECTRON_FEATURES_NPM=	npm yarn1 yarn2 yarn4 pnpm
 _VALID_ELECTRON_FEATURES_REBUILD=nodejs electron