Fix python decorator syntax. (#2827)

1. Python decorators begin a compound statement, so they only appear
   at the start of a line. So match at the line start to avoid giving
   decorator colors to matrix multiplication (@) expressions. Source:
   https://docs.python.org/3/reference/compound_stmts.html#function-definitions

2. Python decorators go to the end of the line and might not include
   parentheses (for example @functools.cache). So instead of matching
   everything until an `(`, just match as many non-`(` characters
   as possible---which both catches the @functools.cache example and
   allows decorator parameters to fall back to the default color.

3. Instead of hardcoding `brightgreen` (which railscast.micro also
   complains about), color decorators as `preproc` (otherwise unused
   by the python syntax files, and arguably the right colorscheme
   group to be using for syntactic sugars anyway). Note this will
   change decorator colors---for example from bright green to kinda
   brown on monokai, and from yellow to more of a light orange on
   railscast.

runtime/syntax/python3.yaml

@@ -20,7 +20,7 @@ rules:
       # keywords
     - statement: "\\b(and|as|assert|async|await|break|case|class|continue|def|del|elif|else|except|finally|for|from|global|if|import|in|is|lambda|match|nonlocal|not|or|pass|raise|return|try|while|with|yield)\\b"
       # decorators
-    - brightgreen: "@.*[(]"
+    - preproc: "^\\s*@[^(]*"
       # operators
     - symbol.operator: "([~^.:;,+*|=!\\%@]|<|>|/|-|&)"
       # parentheses