This commit changes the `offset!` directive so that instead of setting a
`metadata.range` value for the entire pattern, it will set a
`metadata.offset` value. This offset will be applied to the range only
in `vim.treesitter.get_range()`, rather than at directive application
time. This allows the offset to be applied to any and all nodes captured
by the given pattern, and removes the requirement that `#offset!` be
applied to only a single node.
The downside of this change is that plugins which read from
`metadata.range` may be thrown off course, but such plugins should
prefer `vim.treesitter.get_range()` when retrieving ranges anyway.
Note that `#trim!` still sets `metadata.range`, and
`vim.treesitter.get_range()` still reads from `metadata.range`, if it
exists.
