Add a variable `syntax-checking-check-on-buffer-change` to the
`syntax-checking` layer which conditionally sets the variable
`flycheck-check-syntax-automatically` to:
- `'(save mode-enabled)` when nil
- `'(save idle-change new-line mode-enabled)` (the package default)
when non-nil
The various settings determine when Flycheck evaluates the buffer:
- `save` triggers a buffer check after buffer save.
- `mode-enabled` triggers a buffer check when `flycheck-mode` is
enabled.
- `idle-change` triggers a buffer check after the editor has been idle
for `flycheck-idle-change-delay`
- `new-line` triggers a buffer check whenever a new line is inserted
into the buffer
