* Add syntax highlighting for meson build system
It is basically a port of upstream syntax highlighting for vim,
but a bit less noisy (e.g numbers are not colored).
* meson.yaml: fix meson_options.txt detection
* meson.yaml: remove empty rules
* meson.yaml: add highlighting for operators and brackets
* meson.yaml: rearrange the keywords to be in alphabetical order
* meson.yaml: add highlighting for numbers
