ci: Add linting

[abhinav]

This PR adds basic linting to CI for the repository.

First, a preview:

% make help
help:           Prints list of targets and help for them.
build:          Builds all packages.
test:           Tests all packages.
lint:           Runs various analyses on the code.
gofmt:          Makes all files gofmt compliant.
check-gofmt:    Checks that all files are gofmt-compliant.
tidy:           Makes go.mod and go.sum up-to-date
check-tidy:     Checks that go.mod/go.sum are up-to-date.
mdfmt:          Reformats Markdown files with markdownfmt.
check-mdfmt:    Verifies that all Markdown files are properly formatted.

What's included

Currently, linting (make lint) performs three checks:

• code is formatted with gofmt (make check-gofmt)
• markdown files are formatted with markdownfmt (make check-mdfmt)
• go.mod and go.sum are up-to-date (make check-tidy)

Because the behavior of gofmt changes between versions of Go, linting runs in CI only on the latest version of Go.

The make check-* targets above include non-check variants to fix the problems they find. For example, make gofmt reformats Go code, and make check-gofmt verifies that it's formatted properly.

Why the other changes?

This includes two changes besides the GitHub Workflow and the Makefile:

• markdownfmt_test.go wasn't properly gofmt-ed
• README.md was not formatted with markdownfmt

As part of adding these checks to linting, this PR also fixes these issues. So the README.md is now markdownfmt-compliant.

Why no other linters?

The change is already pretty large, so I elected to skip third-party linters for now. A future change could add staticcheck, golangci-lint, etc. if desired.

Refs #30

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Each commit is independently reviewable.

[abhinav]

This will probably conflict with #43 because of the README change. I'll rebase as needed.

[abhinav]

Rebased now that #43 landed.