feat(config): configurable workspace scan extensions and excludes

[AlexanderBrevig]

Closes #51.

Problem

.fs files (F#, GLSL fragment shaders) were unconditionally scanned and indexed as Forth, because the recognised extensions were hardcoded in main.rs with no way to opt out. There was also no way to exclude a folder from scanning.

Change

Adds a [workspace] section to .forth-lsp.toml, using the config machinery that already exists (Config::load_from_workspace):

[workspace]
# Extensions (without the dot) treated as Forth source and indexed.
# Drop "fs" here if it collides with F# / fragment shaders.
extensions = ["f", "fth", "fs", "4th", "forth", "frt"]

# Folder/file names to skip while scanning. Matches a single path
# component, supports `*` and `?` wildcards.
exclude = [".git", "target", "node_modules"]

• extensions — the primary fix for the .fs clash: redefine the set without "fs".
• exclude — skip folders/files; excluding a folder prunes its whole subtree. Matching is per-path-component (not full paths), so a project living under a path that merely contains target isn't wrongly skipped. Kept dependency-free with a tiny */? glob matcher.

Backward compatibility

• No config, or a config without [workspace], behaves as before — the defaults reproduce the historical extension list exactly (case-insensitive).
• Defaults additionally skip .git, target, and node_modules, so scanning is strictly faster than the old "descend into everything" behaviour.

Scope / notes

• This governs the server's workspace scan/index. Whether an editor attaches forth-lsp to an already-open .fs buffer is client-side (filetype → languageId) and out of scope here.
• Config is loaded once at init; this is a static filter, not live-reload.

Tests

5 new tests (252 total pass): legacy-default parity, default noise-dir exclusion + the root-path false-positive guard, extensions override dropping .fs, exclude override with globs, and the glob matcher itself. cargo clippy --all-targets is clean.

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