This commit implements a standalone shared highlighter group that
highlights Jinja statements and expressions.
The traditional way of highlighting file contents is to hook on the
file extension, and assign a custom filetype/highlighter group to
the current buffer. However, since Jinja templates can be based on
any text file format in existence, we do not have a specific file
extension to hook, and consequently, no custom "jinja" filetype.
The user is expected to add the `jinja` highlighter whenever required:
```
require-module jinja
add-highlighter window/ ref jinja
```
Alternatively, file extensions that are known to occasionally pair
with Jinja can be hooked from the user configuration:
```
hook global WinCreate .+\.html %[
try %[
execute-keys -draft \%s \{%|\{\{ <ret>
require-module jinja
add-highlighter window/ ref jinja
]
]
```
The above hook auto-detects statements/expressions (respectively
{%…%} and {{…}} expansions), but will cause false positives
(in terms of highlighting), and therefore isn't part of `jinja.kak`
by default.
