pegs: accept UTF-8 bytes in bare identifier terminals (#25829)

## Summary
- Fixes `std/pegs` lexing for bare UTF-8 terminals such as `\i café`.
- The lexer previously stopped at the first non-ASCII byte, so
`pkTerminalIgnoreCase` never saw the full term despite its rune-aware
`fastRuneAt`/`toLower` matching.
- This now keeps non-ASCII bytes in identifier-style terminals while
ASCII non-ident characters still terminate the symbol.

## Behavior
Before: `match("CAFÉ", peg"\i café")` failed because the terminal was
lexed as `caf`.
After: `match("CAFÉ", peg"\i café")`, `match("Café", peg"\i café")`, and
`findAll` over mixed-case occurrences pass.

`std/pegs` documents `useUnicode = true` as proper UTF-8 support, and
quoted terminals already preserved the same bytes; this makes bare
terminals consistent with that path.

I did not find an existing relevant issue or PR in searches for
pegs/unicode/utf8/getSymbol/pkTerminalIgnoreCase.

tests/stdlib/tpegs.nim

@@ -259,6 +259,11 @@ block:
     doAssert match("EINE ÜBERSICHT UND AUSSERDEM", peg"(\upper \white*)+")
     doAssert(not match("456678", peg"(\letter)+"))
 
+    block:
+      doAssert match("CAFÉ", peg"\i café")
+      doAssert match("Café", peg"\i café")
+      doAssert "two cafés: Café and CAFÉ".findAll(peg"\i café").len == 3
+
     doAssert("var1 = key; var2 = key2".replacef(
       peg"\skip(\s*) {\ident}'='{\ident}", "$1<-$2$2") ==
            "var1<-keykey;var2<-key2key2")