fix(language-core): remove the non-strict configFileName default value

[johnsoncodehk]

When rootDir is passed as /, the default value of configFileName is simply concatenated into an invalid //jsconfig.json, which causes the TS parseJsonConfigFileContent method to perform file searches with incorrect parameters, coincidentally making it impossible to reproduce issue #5598 in posix path systems.

After this PR, I was able to reproduce #5598 on OSX.

It should be noted that for files not included in the tsconfig, rootDir should fall back to the workspace root directory rather than /, which requires investigation in VSCode.

[pkg-pr-new]

Open in StackBlitz

vue-component-meta

npm i https://pkg.pr.new/vuejs/language-tools/vue-component-meta@5606

vue-component-type-helpers

npm i https://pkg.pr.new/vuejs/language-tools/vue-component-type-helpers@5606

@vue/language-core

npm i https://pkg.pr.new/vuejs/language-tools/@vue/language-core@5606

@vue/language-plugin-pug

npm i https://pkg.pr.new/vuejs/language-tools/@vue/language-plugin-pug@5606

@vue/language-server

npm i https://pkg.pr.new/vuejs/language-tools/@vue/language-server@5606

@vue/language-service

npm i https://pkg.pr.new/vuejs/language-tools/@vue/language-service@5606

vue-tsc

npm i https://pkg.pr.new/vuejs/language-tools/vue-tsc@5606

@vue/typescript-plugin

npm i https://pkg.pr.new/vuejs/language-tools/@vue/typescript-plugin@5606

commit: d726130