Update synect crate to 4.5.0.

[christoph-heiss]

So after the issue (and small discussion) in https://github.com/jmacdonald/amp/issues/205 I decided to work on updating scribe (and amp) to the current version of the syntect crate, since 3.0.0 introduced some breaking changes. Although probably not immediately beneficial, having it merged would make updating the syntect crate to future versions much easier. There's one nice thing though - this crate update brings in all new (and updated) syntax definitions bundled as part of the syntect crate.

I post this now to maybe discuss e.g. if we could make this update / the outward facing API of scribe cleaner regarding syntax handling, if we are already introducing breaking changes.

There seems to be no regressions, having used this branch as part of amp daily for more than a week now + all tests are passing.