Fix IDE keyword-match underlining at end-of-line and on multi-byte lines

[fantaisie-software]

Summary

Fixes two related bugs in the IDE editor's matching-keyword underlining, reported at https://www.purebasic.fr/english/viewtopic.php?t=82666 ("PB 6.03 - not work marking of matching keywords"):

• The underline failed to appear when the caret sat right after the last character of a line (e.g. right after EndIf at the very end of a line/file).
• The underline was misplaced on lines containing multi-byte UTF-8 characters (e.g. non-Latin text), because a character offset was used directly as a Scintilla byte offset without conversion.

Changes

• GetWordBoundary() (HighlightingFunctions.pb, and its duplicate in PureBasicDebugger/Standalone_ScintillaStuff.pb, kept in sync per the existing "report to the IDE/standalone debugger" comment on both copies): relaxed the Mode=0 guard from Position < BufferLength to Position <= BufferLength, matching the leniency already given to Mode=1 (autocomplete). The character-scanning logic below the guard already handles running off the end of the line correctly; it just wasn't being allowed to run for this caret position.
• UpdateKeywordHighlight() (ScintillaHighlighting.pb): the "mark the original item" step used StartIndex/EndIndex — character offsets from GetWordBoundary() — directly as Scintilla byte offsets, instead of converting them like every other lookup in the function does. Now uses *OriginalItem's already-correct byte-based Position/Length, consistent with how every other matched/mismatched item is highlighted in the same function.
• JumpToMatchingKeyword() (ScintillaHighlighting.pb): same byte/char mismatch, comparing a parser item's byte Position against a char-based EndIndex. Fixed by converting EndIndex to a byte offset via CharsToBytes() first.

Test plan

• pbcompiler /CHECK /THREAD on PureBasicIDE/PureBasic.pb — clean.
• Full /EXE build of the IDE succeeds.
• Manually verified in the built IDE: placing the caret immediately after the last character of a line (e.g. after EndIf at EOL) now underlines the matching keyword, matching the behavior when the caret is one position earlier.

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