Move LSP installs to ~/.multilspy/lsp/, add server_binary and server_install_dir config

[LakshyAAAgrawal]

Summary

Language servers were downloading binaries into the Python package's site-packages directory (os.path.dirname(__file__) + "/static/"). This is unconventional — site-packages isn't meant to be writable at runtime — and breaks with read-only package installations.

New config options

config = MultilspyConfig.from_dict({
    "code_language": "rust",
    # Point to a pre-installed binary — skips download entirely
    "server_binary": "/usr/local/bin/rust-analyzer",
    # Or override where multilspy downloads server artifacts
    "server_install_dir": "/my/custom/cache/dir",
})

Changes

• Default install location is now ~/.multilspy/lsp/<server_name>/ instead of <package>/static/
• config.server_binary: skip download entirely, use a pre-installed binary at the given path
• config.server_install_dir: override the install location for server artifacts
• Fix Interrupting the rust-analyzer download leads to AssertionError on next invocation #131: check executable existence instead of directory existence (handles interrupted downloads)
• All 10 language servers updated consistently

Server	server_binary	server_install_dir
rust-analyzer	returns binary path	~/.multilspy/lsp/RustAnalyzer/
dart	returns binary + args	~/.multilspy/lsp/dart-language-server/
clangd	returns binary path	~/.multilspy/lsp/clangd/
omnisharp	returns binary, ""	~/.multilspy/lsp/OmniSharp/
typescript	returns binary + --stdio	~/.multilspy/lsp/ts-lsp/
eclipse jdtls	N/A (multi-binary)	~/.multilspy/lsp/EclipseJDTLS/
kotlin	N/A (multi-binary)	~/.multilspy/lsp/KotlinLanguageServer/
jedi	uses as cmd directly	N/A (pip dependency)
gopls	uses as cmd, skips checks	N/A (system binary)
solargraph	returns binary path	N/A (gem dependency)

Fixes #136, fixes #131, fixes #109

[LakshyAAAgrawal]

Credit to @jsimonrichard who proposed the same custom_lsp_binary concept in #125. This PR implements the idea across all 10 language servers and also adds server_install_dir + the install directory migration (#136).

@jsimonrichard — thank you for the original idea and implementation! This PR supersedes #125 with a broader scope. Would you like to review the approach here?