[demo] Use alternate file symbol syntax

[varungandhi-src]

The second commit is the main one. It is currently quite small:

commit 16d82c32c211f38973c78d02578df99c21d7c6ef
Author: Varun Gandhi <varun.gandhi@sourcegraph.com>
Date:   Wed Apr 24 12:59:54 2024 +0800

    Use alternate syntax for file symbols

 src/FileIndexer.ts |  7 +++++--
 src/Packages.ts    | 31 +++++++++++++++++++++++++++++++
 src/ScipSymbol.ts  | 11 +++++++++++
 3 files changed, 47 insertions(+), 2 deletions(-)

I haven't yet made the update for setting file_symbol_info and file_occurrence.

Test plan

Covered by snapshot tests