Implement :source ex command

Running :source {file} currently errors with Command :so[urce] is not yet implemented (PRs are welcome!) because the entry at src/vimscript/exCommandParser.ts uses the undefined stub that falls through to UnimplementedCommand.

Proposal

Implement :source by reading the file and, for each non-blank / non-comment line:

1. Try the three vimrcKeyRemappingBuilder paths (remap, unmap, mapclear) and apply to configuration via the existing public VimrcImpl.{add,remove,clear}RemapFromConfig helpers. This mirrors what src/configuration/vimrc.ts already does at startup so :nnoremap / :unmap / :mapclear work as users expect.
2. Otherwise feed the line through ExCommandLine.parser.tryParse(...) and execute the resulting command. This is the same pattern :foldclose / :folddoopen already use to re-enter the ex-command parser. It naturally supports :set, :edit, :nohl, nested :source, and anything else already implemented.

Path resolution: expand ~ / $HOME, then resolve relatives against the first workspace folder (or the active document's directory when there is no workspace). Cycle detection via a static Set<string> of currently-being-sourced absolute paths, rejecting re-entry with E1092. Missing files report E484.

I have this implemented locally with tests covering basic mapping load, comments/blanks, nested source, cycle detection, missing file, and regular ex-command execution. Full test suite stays green. Will open a PR referencing this issue shortly.

Start at src/vimscript/exCommandParser.ts and compare startup remapping behavior in src/configuration/vimrc.ts. Trace ExCommandLine.parser.tryParse and the existing :foldclose/:folddoopen re-entry pattern, then inspect the issue’s described local tests for mappings, nested sourcing, cycles, missing files, and regular commands. Done means :source resolves paths, applies supported remaps, executes nested ex commands, and reports E1092 or E484 as specified.