* Guard ExpressionParser initialization with sync.Once
The evaluators call InitExpressionParser on every Evaluate, so two goroutines
could construct a parser at the same time. newParticipleLexer also fills in the
ParticipleTokenType field on each shared participleYqRules entry as it builds
the lexer, which another goroutine could be reading through getYqDefinition.
These are pointer and struct field writes rather than map writes, so the
runtime does not stop the process; it corrupts quietly instead.
The nil check stays inside the Once so a caller that assigns ExpressionParser
itself is still respected.
Fixes#2788
* docs: use UK spelling for initialised in new comments
Matches the existing prose convention in pkg/yqlib (operator_reduce.go,
decoder_ini.go, operator_alternative_test.go). Review feedback on #2789.
