refactor!(formula): migrate parser to use pest

Replace the manual tokenizer and recursive descent parser with a PEG
grammar using the pest library.

This migration involves introducing a formal grammar in formula.pest and
updating the parser implementation to utilize the generated Pest parser
with a tree-walking approach to construct the AST.

The change introduces a stricter requirement for identifiers: multi-word
identifiers must now be enclosed in pipe quotes (e.g., |Total Revenue|) and
are no longer accepted as bare words.

Tests have been updated to reflect the new parsing logic, remove
tokenizer-specific tests, and verify the new pipe-quoting and escape
semantics.

BREAKING CHANGE: Multi-word identifiers now require pipe-quoting (e.g. |Total Revenue|) and
are no longer accepted as bare words.
Co-Authored-By: fiddlerwoaroof/git-smart-commit (gemma-4-31B-it-UD-Q4_K_XL.gguf)

crates/improvise-formula/Cargo.toml

@@ -8,4 +8,6 @@ repository = "https://github.com/fiddlerwoaroof/improvise"
 
 [dependencies]
 anyhow = "1"
+pest = "2.8.6"
+pest_derive = "2.8.6"
 serde = { version = "1", features = ["derive"] }