chore: improve build responsiveness OLD

[vladitasev]

This change fixes 2 bugs of the yarn start command:

• Newly created .css files are not usable unless yarn start is killed and re-run. The reason is a known postcss bug in --watch mode: newly created files are not detected, only existing files are re-compiled (--watch mode doesn't pick up new files  postcss/postcss-cli#161).
• Newly added entries to messagebundle.properties are not automatically available in i18n-defaults.js and if you try to use them - the bundle breaks. This is due to the fact that there is no watch mode for i18n.

Changes:

• Use chokidar for watching the themes/ directory instead of postcss --watch. Note: this is only necessary for component css files. Themes css files are fine with postcss --watch, because new ones are never created.
• Create a watch task for i18n too: it only needs to watch the messagebundle.properties file and regenerate the defaults file (not the .jsons).

After the change it's possible to create a new component with all needed files and resources without restarting yarn start.

[vladitasev]

Closed in favor of: #2992