Add Slack Workspaces and Client Sessions Artifacts for Windows

[Gear-I]

Summary

Adds two new artifacts for Slack desktop's Local Storage LevelDB
(Local Storage/leveldb), following the existing Discord/Wire Local
Storage artifact conventions:

• Slack Workspaces (slackWorkspaces) — parses the localConfig_v2
  key into one row per signed-in workspace: team ID, name, URL,
  per-workspace user ID, most-recently-used and version timestamps,
  last-viewed channel/DM ID, and a masked session token.
• Slack Client Sessions (slackSessions) — parses
  activitySession_<teamId> keys into a usage timeline: session
  start, last-activity, and last-logged times, plus a computed
  duration.

Local Storage does not hold Slack message content (that lives in
IndexedDB), so these focus on what it does reliably hold: workspace
enumeration and usage/session timing, independent of message content.

Design notes

• Because Local Storage is a LevelDB, superseded versions of a key
  are preserved on disk. Both artifacts read all recoverable versions
  (not just the live/current one), so a workspace that was later
  removed, or an earlier session, still shows up.
• Session tokens (xoxc-/xoxb-/xoxp-/xoxs-/xoxr-/xoxd-
  prefixed) are masked to the first 8 and last 4 characters — a full
  token is a live credential, not a normal artifact value.
• Cosmetic localConfig_v2 fields (theme colors, sidebar gradients)
  are filtered out of the "Other Fields" column; anything else Slack
  stores per-team is preserved there since it varies by client
  version.
• Both modules are standalone — no shared helper file, no import
  dependencies beyond the existing scripts.chromium.local_storage
  reader already used by Discord/Wire.

Testing

Verified against a real multi-workspace Slack desktop profile
(Slack 4.51.180.0, Windows). Confirmed correct output for 3 real
workspaces and their usage sessions, including token masking and
timestamp conversion from both ISO 8601 and epoch formats. pylint
clean (10/10) on both files, no pylint: disable suppressions.

Sample data

Both artifacts declare a sample_data entry for corpus
slack_win_multiworkspace (Slack 4.51.180.0, Windows):

• slackWorkspaces: 5 rows
• slackSessions: 4 rows