Centralize package metadata in the workspace `Cargo.toml` and inherit
it in sub-crates. Add license symlinks and README symlinks to the
workspace root. Set `documentation` URLs for library crates.

syntax

This introduces a workspace restructuring with separate library and
proc-macro crates, replacing the old declarative macros with a new proc
macro that supports simplified call syntax, implicit template variable
names, and mixed simple parameters with implementation blocks. Display
blocks (`??? >>> name` / `??? <<<`) provide conditionally included
sections with per-arm control, enabled by inserting a param key. The old
`macro_rules!` macros, deprecated module, and standalone test files are
removed in favor of comprehensive inline tests.

Move the old `tmpl!` and `tmpl_param!` macros into a dedicated
`just_template_macros` proc-macro crate. The new `tmpl!` macro supports
both simple parameter assignment and multi-arm implementation blocks
with per-arm overrides. Remove the deprecated `deprecated` module and
update tests accordingly.

Deprecate old macros in favor of new deprecated module, replace the
external file-based test with inline unit tests, and introduce the
`??? >>>` / `??? <<<` display block syntax for conditional content
inclusion per arm or globally.

Introduce a CHANGELOG.md file and update the `tmpl!` macro to use a
cleaner syntax with comma-separated key-value pairs instead of the `+=`
operator and parentheses-wrapped parameters

Fix empty impl block generating unnecessary newlines