Closes#1954
Plugin file templates could only take a typed name plus a fixed extension, so dot-files such as .dockerignore and templates needing more than a name weren't expressible. Templates (API ^1.3) can now declare a fixed `fileName` and named `variables` that the New File dialog asks for, and plugin templates get the same Java/Kotlin/Go/PHP package inference as built-in ones. Unsafe filenames and existing files are refused. The dialog no longer submits on Enter while an IME is composing, and its variable form has proper padding. Adds a file-templates test fixture.
Co-authored-by: arnaud-ritti <[email protected]>