Allow tab-indented EML templates

[funwithcthulhu]

Summary

• Treat tabs as leading EML whitespace when detecting the start of template text.
• Add tokenizer coverage for a tab-indented template line.

Fixes #392.

Validation

• git diff --check
• opam exec -- dune build src/eml/main.exe
• opam exec -- dune build src/eml/eml.cma src/eml/eml.cmxa
• opam exec -- dune exec src/eml/main.exe -- --stdout <tab-indented temp file>

Blocked locally:

• opam exec -- dune runtest test/expect/eml needs ppx_expect.config_types and ppx_expect, which are not installed in this switch.
• opam exec -- ocamlformat --check src/eml/eml.ml test/expect/eml/tokens.ml needs ocamlformat 0.25.1; this switch has 0.29.0.