fix: keep trailing underscore in expression before close tag

[spokodev]

What

An expression whose last token is an identifier ending in _ is truncated when the close tag immediately follows it, because _%> is matched as a substring with no left boundary.

ejs.render('<%= foo_%>   END', { foo: 'WRONG', foo_: 'RIGHT' })
// before: "WRONGEND"  -> read `foo`, then slurped the trailing spaces as `_%>`
// after:  "RIGHT   END"

The spaced form already behaves correctly, which shows the truncation is unintended:

ejs.render('<%= foo_ %>   END', { foo: 'WRONG', foo_: 'RIGHT' })
// "RIGHT   END"  (both before and after this change)

The slurp rule

_%> is the documented whitespace-slurping close tag ("removes all whitespace after it"). It is only a close tag when its leading _ acts as a delimiter prefix. When the _ continues a JavaScript identifier (foo_), it belongs to the expression, not to the close tag. The -%> newline-slurp close is not affected by this because - is not a valid identifier character, so foo-%> is already unambiguous; _ is the one slurp-prefix that is also an identifier character.

Root cause

Two places treat _%> as a close with no left boundary:

1. The tokenizer alternation in createRegex (...|%>|-%>|_%>). During parseTemplateText, pat.exec finds the leftmost match, so for foo_%> the _%> alternative matches at the _ (index 4) and beats %> (index 5). The expression content becomes foo and the close becomes _%>.
2. The whitespace pre-pass that textually strips spaces and tabs after a _%> close, which also matched foo_%> and stripped the trailing whitespace.

Fix

Guard the _%> close in both places with a (?<![$\w]) lookbehind so it is only treated as a close when the _ is not preceded by a JS identifier character. The lookbehind is zero-width, so parseTemplateText's index/slice logic is unchanged. In createRegex the guard is added after the delimiter substitution so the </> replacements do not rewrite the lookbehind syntax, which keeps custom delimiters working.

Tests

Added to the existing <%_ and _%> suite in test/ejs.js:

• <%= foo_%> END now reads foo_ and keeps its whitespace, matching <%= foo_ %> END.
• A real _%> still slurps: <%_ var x = 1; _%> and <%= 1 _%> are unchanged.

Reverting only the source change makes the new truncation test fail (red) while the slurp-intact test stays green. With the fix, the full suite passes (168 tests), including the custom open/close delimiter test.