feat: Support parsing of SQL code blocks in Markdown files

[michael-the1]

Closes #593

There are two caveats that would be good to consider:

1. Because we parse to AST -> rebuild from AST, we can actually lose some trivial whitespace characters. mistletoe keeps all the important whitespace, but can toss out some unnecessary whitespace, such as trailing whitespace or double newlines. A good example is in the README.md of this very repo:

 ~/sqlfmt
❯ poetry run sqlfmt README.md
1 file formatted.
0 files left unchanged.
README.md formatted.

 ~/sqlfmt
❯ git diff
diff --git a/README.md b/README.md
index 7d292f9..5a3f457 100755
--- a/README.md
+++ b/README.md
@@ -9,7 +9,7 @@


 sqlfmt formats your dbt SQL files so you don't have to. It is similar in nature to black, gofmt,
-and rustfmt (but for SQL).
+and rustfmt (but for SQL).

 1. **sqlfmt promotes collaboration.** An auto-formatter makes it easier to collaborate with your team and solicit contributions from new people. You will never have to mention (or argue about) code style in code reviews again.
 2. **sqlfmt is fast.** Forget about formatting your code, and spend your time on business logic instead. sqlfmt processes hundreds of files per second and only operates on files that have changed since the last run.
@@ -138,7 +138,6 @@ Config for other integrations is detailed in the docs linked below:
 - [SQLFluff](https://docs.sqlfmt.com/integrations/sqlfluff)
 - [VSCode](https://docs.sqlfmt.com/integrations/vs-code)

-
 ## The sqlfmt style
 The only thing you can configure with sqlfmt is the desired line length of the formatted file. You can do this with the `--line-length` or `-l` options. The default is 88.

I think this is acceptable since it does not affect readability and should not affect output (i.e. when it's rendered to HTML).

2. Nested sql blocks are not formatted. I think this is acceptable. See example:

``````markdown

# Hello

```sql
SELECT 1 -- this is not formatted
```

``````

[tconbeer]

Thank you for this! I'm mostly away from my computer this week but I'll try to review next week.

[tconbeer]

Wow, thank you!

Just a couple of nits, honestly. Really well done with this. I really, really appreciate the tests in particular. GHA is down right now, so the checks won't run, but assuming those pass I think we can get this merged really soon.

[tconbeer]

Let's wrap this in try ... except ImportError and either return the source or re-raise a SqlfmtError to create a nicer error message for users who don't have the extra installed. I know right now we shouldn't hit that codepath due to the Mode property, but would be good to safeguard against that changing in the future.

[michael-the1]

I added the try ... except guard. I went ahead and subclassed SqlfmtError since I didn't see any bare raise SqlfmtError anywhere.

[tconbeer]

@michael-the1 Sorry I dropped the ball on this. I'd love to get this finalized. Looks like there are quite a few failing tests and lints. Do you want to work through those? Let me know if you get stuck.

[michael-the1]

@tconbeer Life can get busy which I totally understand.

The reason the tests are failing is because mistletoe is not installed in the test workflows. Should I add it to the dev or test group in pyproject.toml? Feels a bit... not neat 😅

[michael-the1]

Ah, I see now that, in the Github workflow, dependencies are installed with poetry install --sync --no-interaction -E jinjafmt --only main,test. I'll change this to also install markdownfmt.

I also see some other errors in the static analysis and the primer workflows. I'll take a look at those tomorrow.

[michael-the1]

@tconbeer I think I fixed all issues, but with two notes:

1. mypy was giving some errors due importing mistletoe.
   • mypy --disallow-any-unimported will fail with the new mistletoe import. I removed the option from the Makefile.
   • I also set ignore-missing-imports for mistletoe (see docs). The alternative is to change the type hints to Any and work from there, but it's not my preference since we'd change the code and hurt readability just to wrangle with mypy.
2. The code coverage check was failing, but I think it's related to a repository setting - something with the CC_TEST_REPORTER_ID env variable not being set (see workflow logs)