Add detection of Snakefile

[HDembinski]

Copied from atom/atom#20382

Description

https://snakemake.readthedocs.io/en/stable/ is a popular tool in data science. It parses so-called "Snakefile"s which are similar to "Makefile"s.

The grammar selector detects a "Snakefile" as source.python, but it should be source.yaml. This causes issues with linters and autoformatters like python-black, which should not be run for Snakefiles.

Steps to Reproduce

1. Generate a file called "Snakefile" with the following content:

import glob

rule all:
    input: ["data/foo.root", "data/bar.root"]

rule make_foo:
    input: "{name}.root"
    output: "{name}_pv.root"
    shell: "./make_foo.py {input} {output}"

2. Open file in Atom, it is detected as python.source.

Expected behavior:

The grammar selector should detect a "Snakefile" as source.yaml.

Actual behavior:

The grammar selector detects a "Snakefile" as source.python.

Reproduces how often:

Always.

Versions

Atom : 1.43.0
Electron: 4.2.7
Chrome : 69.0.3497.128
Node : 10.11.0

[Nixinova]

Are Snakefiles not meant to be Python? Search shows that all of them contain python code, and your example includes an import statement.

[HDembinski]

They are neither yaml nor Python. Look closely at the code example, this is not valid Python syntax.