Add MPL license headers and CI check

[kewisch]

MPL is a per-file license, so applicable source files need to include the appropriate license header.

Implement:

• A reusable script to add the MPL license header to relevant files. It should support exclusions for files/paths where the header should not be added.
• A CI check that ensures relevant files have the required license header and fails clearly when they do not. Can focus on only changed/added files if performance is a concern.

The solution should be straightforward to use across our repositories and allow repository-specific exclusions where needed.

[devmount]

@kewisch Since we need this cross-repository and want to prevent several script versions or CI workflow versions spread in different repositories, we should aim for a centralized/versioned way of handling that.

So here's my take on a possible solution, please let me know what you think:

Reusable GitHub workflow

We create a reusable workflow file here in the pro repo, e.g. license-check.yml. Other repositories can then add that workflow as a job to their (maybe already existing) validation workflows with:

jobs:
  license-check:
    uses: thunderbird/pro/.github/workflows/license-check.yml@main

This should be minimal invasive for other repos and we can maintain the script / have a single point for truth. When we are ready, we should maybe pin to a tag (e.g. @v1) instead of @main, so a change in thunderbird/pro can't unexpectedly break CI across every repo using this at once.

Script for actual checking / completing source files

We create a bash script add-license-headers.sh inserting the MPL header into any relevant file that's missing one. We can give it a --check flag to only report without changing something. Per default it could scan every tracked file (git ls-files) but we can also make it accept a REF to check against (git diff --name-only REF...HEAD) or a static list of file paths.

Configuration

I suggest we create two additional config files, which each repository can decide to override on their own if they wish (otherwise they'd just fall back to the default config):

.license-ignore

An exclusion list in gitignore-syntax the script will not check or add license headers to. Example:

# Dependencies / vendored code
node_modules/
vendor/
.venv/
venv/

# Build / generated output
dist/
build/
*.min.js
*.min.css

license-comment-styles.conf

A file extension to comment style mapping list, for repositories to define the actual comment syntax the license should show up. We could also skip/automate this, but I think having the option to override for some exotic files we might have is helpful. Example (I listed the most common file types from our repos that might be suitable):

# Styles: slash (//), hash (#), block (/* */), html (<!-- -->), jinja ({# #})

js      slash
jsx     slash
ts      slash
tsx     slash
mjs     slash
cjs     slash

py      hash
sh      hash
bash    hash
yml     hash
yaml    hash
ftl     hash

css     block
scss    block
less    block

html    html
htm     html
vue     html

jinja2  jinja
j2      jinja