feat: add first-non-blank motions for ^, _, and I

[jtams]

This PR adds support for the I, _, and ^ Vim motions, all of which operate on the first non-blank character of a line.

^ was previously mapped to Home, which moves the cursor to the beginning of the line rather than the first non-blank character. This PR fixes that behavior.

It also adds support for I and _. The _ motion supports counts; for example, 3_ moves the cursor to the first non-blank character of the third line, equivalent to 2j^ in Vim.

Also adds tests covering the new behavior.

[jtams]

This PR resolves #18

[oribarilan]

Heads up: #62 just merged, which restructured the vim engine from a single src/vim.ts into a modular src/vim/ tree. This PR will need a rebase onto main, and the code you're touching has moved:

• motion handling → src/vim/normal.ts
• motion tables (MOTIONS, SELECT_MOTIONS, DELETE_MOTION) → src/vim/tables.ts
• tests → test/vim/normal.test.ts

Also note CI is now stricter: Biome runs with noExplicitAny, noConsole, and noNonNullAssertion as errors via just check. Happy to help with the rebase if useful.

[jtams]

I rebased onto main and ported the changes to the new modular vim engine. Thanks for the heads up.