[nfc] Add legacy syntax support to TextMate grammar

Add support for three legacy FIRRTL syntaxes that are no longer in
the specification but may still appear in older code:

1. Legacy connect: 'foo <= bar' (equivalent to 'connect foo, bar')
2. Legacy partial connect: 'foo <- bar' (no modern equivalent)
3. Legacy invalidate: 'foo is invalid' (equivalent to 'invalidate
   foo')

All three legacy operators are highlighted with the scope name
'keyword.operator.legacy.firrtl' to distinguish them from modern
syntax.

Additionally, fix several issues with layer and control flow
syntax highlighting:

- Fix 'enablelayer' on modules to support multiple occurrences
- Improve visibility of 'layer' and 'layerblock' keywords
- Fix 'else when' syntax to properly highlight expressions
- Update layer pattern to match 'bind'/'inline' conventions
- Simplify layerblock pattern to match actual syntax

AI-assisted-by: Augment (Claude Sonnet 4.5)
Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

Author: seldridge