feat(chat): add `include` role to include files

Files may be included in the chat by a special `include` role. Each
file's contents will be added to an additional `user` role message with
the files separated by `==> {path} <==` where `{path}` is the path to
the file. Globbing is expanded out via `glob.glob` and relative apths to
the current working directory (as determined by `getcwd()`) will be
resolved to absolute paths.

Example:

```
>>> user

Generate documentation for the following files

>>> include

/home/user/myproject/src/../requirements.txt
/home/user/myproject/**/*.py

```

Fixes: #69

1 files changed, 1 insertions, 0 deletions

diff --git a/syntax/aichat.vim b/syntax/aichat.vim
index ecd8459..1f32efb 100644
--- a/syntax/aichat.vim
+++ b/syntax/aichat.vim
@@ -1,5 +1,6 @@
 syntax match aichatRole ">>> system"
 syntax match aichatRole ">>> user"
+syntax match aichatRole ">>> include"
 syntax match aichatRole "<<< assistant"
 
 highlight default link aichatRole Comment