fix: parse adjacent set_by_lua_block opening brace

[wow220809]

Proposed changes

Fixes #177.

The Lua lexer previously collected the set_by_lua_block output variable
until it encountered whitespace. As a result, when the opening brace was
adjacent to the variable, such as $pass{, the lexer consumed the brace as
part of the argument and later failed while looking for the Lua block opener.

This change:

• Treats an unquoted { as the end of the output-variable argument and the
  start of the Lua block.
• Preserves ${variable} parameter expansion by not treating the first {
  after $ as the block opener.
• Reuses the consumed opening brace as the initial Lua block depth.
• Adds regression coverage for both spaced and adjacent opening braces with
  $pass and ${pass} forms.

This aligns the Lua lexer with the NGINX configuration tokenizer, where an
unquoted opening brace terminates the current argument and starts a block.

Validation

• go test ./... -run '^TestLex_setByLuaBlockOpeningBraceDelimiter$' -count=1
• go test -race ./... -count=1
• go vet ./...
• gofmt -d lua.go lex_test.go
• golangci-lint v1.64.2 with new-issues-only checking

Checklist

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md) -- no documentation change is required for this lexer-only bug fix